The opinions amongst historians amateur and professional range from economic and military to accusations of conspiracy.
'Traditional' Capitalist Interpretation
Economic Liberals argue that the fall of the USSR is purely economic and is yet another piece of evidence that Communism does not work as an economic model. They cite the failure of the collectives, pointing out the fact that the USSR had to import grain from the US, much to the motherland's great embarassment that they found ways to import through Canada instead. They also cite that Communism lacks incentives for entrepreneurs and innovators to thrive, identifying the fact that the Soviet TMT (technology, media and telecommunications) industry was light years behind the US due to lack of private incentives for innovators to improve Soviet technology. Lastly, they point that the USSR's industry in fact relied on discounted imports from their clients (note: questionable source I know, but I couldn't find anything else on COMECON). When these discounts dried up due to the collapse of their client states, so did their economy, which led to the implosion.
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.
The epitome of this is in the 1980's, when 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 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.
Nationalist Russian Interpretation
Nationalist Russian historians will have arguments familiar to any nationalist historian: blaming outside factors rather than the Soviet system itself. Their arguments usually pin Gorbachev as a western agent which engineered the downfall of the USSR and label him as a traitor to the motherland. Like all nationalist arguments, they usually ignore the facts and realities on the ground and go by hearsay and rumours.
There are other interpretations, but I'm not as familiar with them.