Mikhail Gorbachev himself inadvertently contributed to the fall of the Soviet Union. His policies of Perestroika (economic reforms, bringing the communist economy close to China's modern Communist-Capitalist system) and Glasnost (free speech, which had been banned for years in the USSR) brought the society further and further from Lenin's original, and further and further toward a more Western approach. Whilst this made him popular across Eastern Europe, and indeed the world, he never meant for these reforms to cause the fall of the USSR and Communism in Eastern Europe.
Western aggression was also at it's height from 1979, when Jimmy Carter was inaugurated. Ronald Reagan - who vocal opposed the Soviet regime - came into office in 1981. He was a strong supporter of 'Star Wars' (or the Strategic Defense Initiative), which was designed to disarm Soviet Missiles before they hit, thus making Nuclear War theoretically unwinnable for the Soviets. Reagan also aimed to cut the Soviet Union off economically, driving oil prices to their lowest in centuries. Without the great income of their oil industry, the Soviet economy began to crumble.
Whilst talking about economy, it is significant to mention that Stalin's Five Year Plans were entirely focused on producing capital goodsin order to promote industrialization in the USSR and allowing them to progress into a global superpower. However, this meant a lack of consumer goods, and famine swept across the USSR. This never really changed, meaning many Soviet citizens were periodically below the breadline. Living standards were often appalling. This was awful propaganda for Communism, and undermined the idea of economic superiority in the Soviet system, causing cries for revolution.
Hope this helped! Cold War politics are fascinating :) x