To add an alternative explanation to Evil Washing Machine's excellent list of explanations, I would append:
Abandonment of terror
Coercion, that is, coercion of internal dissent and coercion of subject states, was an inherent and vital part of the Soviet system. Remove the coercion, as occurred in the 1980s, and the system's collapse becomes much more likely if not inevitable.
Removal of coercion as an element of foreign policy
Soviet foreign policy before 1980 was dominated by the so-called Brezhnev doctrine, which essentially said the Soviets would intervene militarily rather than allow a country to fall out of the communist orbit. This doctrine was enforced until the early 1980s, when the Soviet leadership refused to intervene militarily in Poland to ensure the survival of the Polish communist state against domestic unrest. The Polish communist state survived that unrest, but the Brezhnev doctrine was dead, and the Polish crisis then was just a foreshadowing of future unrest in Warsaw pact states to come, when the Soviets under Gorbachev would also refuse to intervene militarily, making the collapse of the Warsaw pact that much more likely.
And the collapse of the Warsaw Pact was disastrous for the survival of the Soviet Union itself for a number of reasons:
- Nationalist revolutions in the Pact inspired Nationalist revolutions inside the USSR itself (similar to how revolutions in the Arab Spring inspired further revolutions)
- The Soviet Union depended on these states for imports and exports, as alluded to by Evil Washing Machine in his section on economic decline. The removal of these states from the Soviet orbit threatened the stability of the Soviet economy.
Removal of coercion as an element of domestic policy
Gorbachev, as is well known, was unwilling to leave the hard line attitude towards internal dissent in place. Glasnost and Perestroika made criticism of the regime much more tolerated, and this emboldened people across the various member republics to do exactly that. If know that they are going to be able to push the limits of criticism, they are going to do exactly that. Inspired by thee aforementioned unrest elsewhere, economic hardship, revolutions in the Warsaw Pact, the soviet failure in Afghanistan, etc. elements of Soviet society at the top and at the bottom, in Russia and the other member republics, began agitating for serious change and the entire Union was destabilized.
I would argue this did not necessarily mean the collapse of the Soviet Union, and its noteworthy to point out that many of the Soviet member states actually voted to maintain a reformed version of the Soviet Union but without successful reforms away from a one party dominated dictatorship, the Soviet state was doomed to fail the moment its leaders refused to coerce their own people and their subject states.