This is a very broad question but I will try to summarize it simply into a few paragraphs. Of course, I will probably be lynched by this, as the consensus ranges from economic and military to accusations of conspiracy.
Military, Economy, and Openness/Restructuring
In short, all 3 factors. Reagan and Thatcher's increased belligerence towards the Soviet Union caused an arms race which the significantly weaker Soviet economy could not keep up. This then wrecked havoc on the Soviet public as light industries were switched to heavy industries (yes I know it was going on already, but one could argue that it was even more prevalent in the 80's) and public spending was tapped off for defence spending. The high number of complaints (for example, Radio Yeveran were very popular in the Soviet states) then led to Gobrachev's policy of Glasnost and Perestroika which precipiated the downfall of the Warsaw Pact.
What is significant
The debate amongst historians is therefore not what the cause is, but which is the most significant. Economic Liberals will argue that it is another case of the failure of communism, citing slower Soviet output due to the lack of incentives, lower grain production such that the USSR had to import grain from the US, much to their embarassment and the lack of development in Technology, Media and Telecommunications due to again, lack of incentives.
Anarcho-Liberals and Marxists would argue that the US simply harnessed its much greater economic power (often with mentions that this power was gained through worldwide imperialism and forcing states in, say, post war Europe for example, to buy their goods) to simply outspend the USSR in conflicts and military might. They will cite how USSR's GNP rose steadily from the 40's to 80's until declining as more development was being focused in weapons technology and heavy industry; something that a country wrecked by WW2 cannot afford to do so soon after the war (given the scale of destruction the USSR faced, 'soon' is an appropriate term) whereas the US, untouched by carpet bombing and enjoying the post war boom due to lack of competition, can do freely.
There are other interpretations, but I'm not as familiar with them.