The extent was:
17 drivers were murdered and 1,500 buses destroyed. […]
With the introduction of internment in 1971, the situation deteriorated further. Violence increased dramatically with gun attacks and bombings taking place on a scale never before experienced.
Bus drivers were no longer safe from being attacked and also robbed. Whole streets were burned down. Both sides now seemed hell-bent on destroying public transport.
— Eugene O’Callaghan: "Bus heroes who steered us through the Troubles", Belfast Telegraph, September 21 2019.
How that 'tradition' escalated is described in a whole book on this specific topic:
During the IRAs border campaign of 1956–62, the railway network as well as buses were regarded as legitimate economic targets and a number of UTA vehicles were damaged or destroyed.
However, none of the above compares with the level of destruction caused during the current period of civil unrest in the province. This began in the mid-1960s and, at the time of writing, has not completely ceased. During nearly four decades of the current troubles, bus workers have been injured or even killed and hundreds of buses to the value of millions of pounds, as well as depots and other facilities, have been damaged or destroyed. If all the vehicles destroyed between 1964 and 1998 could be lined up nose to tail, they would stretch from the City Hall in Belfast to Lisburn a distance of 8½ miles.
The managers and staff of Ulsterbus, the Belfast Corporation Transport Department and later Citybus have had to try to provide transport services to the public of Northern Ireland in conditions not experienced anywhere else in the UK with the possible exception of brief periods during the Second World War in British cities subjected to sustained air attack.
— Michael Collins: "Buses Under Fire: Northern Ireland's Buses in the Troubles", Colourpoint Books: 2006.
One interesting detail in this might be the fact that this was not purely wanton destruction of infrastructure. Apart from delegitimising the state and its monopoly on violence, at the same time a parallel society was conjured up to compensate for this:
The morality of the IRA went beyond an abhorrence of criminal behaviour. At a number of different levels the IRA have attempted to provide community services for the people living in their areas. For example, in Belfast the IRA established a large taxi company to ferry people to and from nationalist areas in the city. The profit the IRA made from the taxis was very modest, as the fares were considerably lower than those on public buses. However, as one police officer explains, the IRA did not operate the taxis purely for money:
[the IRA] are not prepared to put their prices up to match the buses, as they have taken a decision that what they are providing a service to the community and, as such, they should continue to subsidise it if possible.'
— Andrew Silke: "The Lords of Discipline: The Methods and Motives of Paramilitary Vigilantism in Northern Ireland", Low Intensity Conflict & Law Enforcement, 1998.
Curious details from the beginning that also might have led to such a focus on buses:
When Ian Paisley led a march through Derry in late November 1968 in his first political appearance in the city, the parade consisted entirely of bands and groups from outside Derry. The marchers were brought to the city in buses and a ‘special’ train. They were applauded by groups of local Protestants, of which the largest group was gathered at the edge of the Fountain, but none of the platform speakers came from anywhere closer than Coleraine, thirty miles away. The Reverend Charles Tyndall, the Church of Ireland Bishop of Derry and Raphoe, a noted liberal, referred to the march as an ‘invasion from Belfast’. […]
An incident in the first few days after the arrival of the troops illustrated how far the army was prepared to go to maintain good relations in this initial period. During the ‘Battle of the Bogside’, a brand new bus had been hijacked. It had ended up as the centrepiece of a formidable barricade in Rosemount where it provided resting quarters for peace corps members on duty at the barricade.121 The bus company was concerned to get the bus back and approached the army. A compromise was worked out through the army whereby the bus company would provide an older bus in exchange for the return of the new model being used in the barricade. Lt Col Millman, commander of the 1st Battalion, Queen’s Own Regiment, said that if this arrangement did not suit the DCDA the army would provide alternative materials for the barricade and, if necessary, the army would rebuild the barricade themselves. […]
At 11 p.m. the following night, some distance from the usual rioting grounds at William Street, a crowd of about 150 youths commandeered two buses and built a barricade at the edge of the Brandywell. Once again the army resisted responding. […]
It was only when the gathering crowd set fire to the buses an hour and a half later that the army responded. If there had been some sort of agreement between the army and the DCCC it was suspended now. After an interval the troops began firing CS gas at a crowd several hundred strong, which, according to the Derry Journal, was composed mostly of ‘anxious lookers-on, fearful for the safety of their families and property’. Rioting then broke out in the William Street area some distance away and the troops arrested a dozen people there. In the course of these riots, six shots were fired at troops in Bishop Street, though there were no injuries. The bullets were of three different calibres, suggesting the use of three different guns.
Throughout this time there were regular large-scale riots in Derry and in early February 1971, during a riot, a bus was hijacked by three masked men, one armed with a pistol. It demonstrates the ever closer involvement of Republicans and IRA members in initiating and participating in rioting. By early 1971, the Provisional Republicans were very definitely on the offensive in Derry. […]
Criticism of army conduct in their regular raids into the area became even harsher. The two tenants’ associations in Creggan had been involved in negotiations to get Ulsterbus bus services restored and to get street lighting and building work resumed in the area. ‘All organisations’ in the area (i.e., including the two IRAs) had agreed that barricades should be removed to allow bus services to resume, according to the tenants’ associations but, they said, ‘it appears that this does not suit the army’ who launched a massive ‘search operation’ at this sensitive time. The tenants’ associations said they ‘viewed this action as sheer provocation’. During one of these raids in November, which the army described as a ‘small arrest operation’ by 200 troops (during which no-one was arrested), a middle-aged housewife, Kathleen Thompson, who had come out to raise the alarm, was shot dead in her garden by a single shot fired by a soldier. Creggan and Foyle Hill tenants’ associations issued a joint statement saying that the army had ‘replaced harassment with intimidation and murder’. […]
The influx of new members created tensions with some of the older Republicans. In mid-February 1972 the Provisional IRA in Creggan kidnapped Thomas Callaghan, a middle-aged bus driver who was an off- duty UDR member. They then killed him. It was the first killing in Derry of a local off-duty member of the security forces. There was widespread local condemnation and some of the older Derry Republicans conveyed their criticism of this action and of the ‘arrogance’ of the younger IRA members to the Dublin leadership. […]
As the commercial centre of Derry was blown up systematically and the city became a dark bleak place, Catholics turned to the towns and villages just over the border in Donegal. In a direct reversal of the usual pattern, people from this provincial city began to travel in large numbers to outlying towns up to twenty-five miles away for a huge variety of reasons. Above all, the social life of huge sections of the Catholic community in Derry began to take place on the other side of the international border. There had been a longstanding pattern of both Catholics and Protestants from Derry crossing the border to go to the beach or play the slot machines, or belonging to golf clubs or having caravans in Donegal. However, with virtually every dance-hall or disco bombed in Derry, buses, taxis and cars began to ferry thousands of teenagers out to discos and bars in Moville, Buncrana and even twenty-five miles away to Letterkenny every Friday, Saturday and Sunday night. The social life of the vast majority of young Catholics was being conducted on the other side of the border, a situation which was reversed only in the late 1980s as new discos and nightclubs were opened in Derry.
— Niall Ó Dochartaigh: "From Civil Rights to Armalites. Derry and the Birth of the Irish Troubles", Palgrave Macmillan, 2005.
If the above gives rise to suspicion that only the Cathoic side used this 'tool', after the introductory above that "both sides now seemed hell-bent on destroying public transport":
The two men concerned were active in an East Belfast UFF unit and were detained without trial after a grenade attack on a Catholic workers’ bus. […]
Then the UDA began to emerge in force as the responsibility for making a reality of the strike shifted increasingly on to Tyrie’s broad shoulders. Buses all over Belfast were hijacked to make roadblocks and picketing was intensified. ‘The day that the workers called the strike, people went out to work,’ Tyrie wrote much later, ‘so I spoke to the UDA to devise a method to stop them. […]
He [Bob Pagels] continues to work in the Island Centre for the Handicapped: ‘I’ve been organising some buses for the people we look after here,’ he told the author. ‘There you are, back in 1974 one of my jobs was hijacking buses to build UDA barricades. […]
[…] and replacing burnt-out buses and public buildings added another £7 million to the bill.
Stone-throwing attacks by Protestant pupils injured several girls from Our Lady of Mercy Secondary School as their bus passed up the Ballysillan Road on 3 October. […]
— Ian S. Wood: "Crimes of Loyalty. A History of the UDA", Edinburgh University Press, 2006.