To the extent that the fall of the Soviet Union could be considered inevitable because of the fragility and artificiality of its economic system, then its demise can be traced to the collectivist policies enforced at the cost of millions of lives during the Stalin years. The Soviet economy, by design, ignored fundamental laws of supply and demand. Instead, the highest level of the government determined how, when and where resources would be utilized. Consequently, the economy, while very large, was also very precarious. Combined with the deep divisions within the Soviet Union, which included more than 100 different ethnic groups and languages, the Soviet Union can be considered to have been a house of cards, with its collapse predetermined.
I suspect, however, that the question is geared more toward the final years of the Cold War and of the Soviet Union. By the 1980s, the vast economic and social disparities between the Soviet Union and those countries in Central and Eastern Europe under its control on one side and the prosperity and cultural freedoms in the United States and its allies in Western Europe and Asia on the other side became too great for the governments of the former to survive.
The major events leading to the final collapse of the Soviet Union were the election of President Reagan and and deaths of successive Soviet leaders, beginning with the 1982 passing of long-time General Secretary of the Communist Party Leonid Brezhnev. President Reagan, with support from allied governments in Western Europe, launched a political offensive against the Soviets that stood in marked contrast to the foreign policy of the preceding Carter Administration. The Reagan Administration also began a major expansion of U.S. military capabilities, including the announcement of a space-based system intended to defend the United States from Soviet long-range missiles.
While the U.S. position in Europe was being strengthened, the Soviet Union's new leader Mikhail Gorbachev, a much younger and more liberal member of the ruling Politbureau of the Communist Party initiated a series of policies intended to fundamentally restructure the Soviet economy while opening the country up politically. These policies, known as "perestroika" (the restructuring of the economy) and "glasnost" (lifting restrictions on freedom of expression) facilitated an unprecedented liberalization of the Soviet Union. While the Russian people of the Soviet Union enjoyed their new-found freedoms without fear of the secret police, the many other nationalities within the Soviet Union began to agitate for independence.
The Soviet Union was a vast empire formed and maintained through military force and the ever-present secret police. Once the threat of force was removed, those nationalities in Central Asia and the Caucasus were able to assert their independence. In some instances, for example, the Republic of Georgia, the separation was violent and bloody. In other cases, for example, most of the Central Asian republics, the separation was relatively peaceful.
In conclusion, the major events leading to the end of the Cold War were the election of an assertive American administration and the rise to power of a liberal member of the Soviet Communist Party.