There are 3 people primarily who were the cause: Russian Nikita Kruschev, our Prez Jimmy Carter, and their Premier Gorbachev.
Kruschev held a major trade fair in Moscow, in 1959. He thought this would be a major accomplishment for Russia, to have a major world event held there. Instead, Russians got a taste of what a typical American home was like...and, they became very envious. The USSR had been a very poor country since the Communist revolution (partly because they spent so much on their military), because Communism was a very inefficient economic system. The leaders got around discontent by promising people things would 'get better soon', but after Russians saw what it was truly like to live in America; then the USSR had goals that were actually impossible to reach.
Forward to the late 1970s. Prez Jimmy Carter decides to arm rebels called 'mujahadin' in the Communist nation of Afghanistan; in retaliation Russia invades, and gets mired in a war very similar to out Vietnam. After a horrible mess, the Russians leave in disgrace; appearing to be a well-trained high-tech army that was beaten by a few guerilla soldiers. There's now a permanent image of Russian military weakness.
Carter also sets up the Camp David Peace accords, between Egypt and Isreal; eventually leading to today's Egypt-Isreali alliance, and major US military aid to Egypt. Through the peace accords, Russia was pushed out of a leadership role in the crucial Middle East, where till now she was leading against Russia. The US then offers military aid to the Saudi Arabian royal family; completing the pattern of pushing the USSR out of the Middle East.
Premier Gorbachev then comes to the fore, as the USSR is so desparate; it is seeking ideas from anyone. This is the policy called 'glastnost' or 'peristroika.' Open criticism of the USSR is encouraged (as a way to help the USSR improve), but it seems the criticism becomes more than the government can handle. The critical point comes when the government loses control of the media.
Gorbachev then has the only alternative of seeking political reform, based on what his own people have told him. Which signals the call for world-wide Communists to reform greatly, or step aside--eg., the successful anti-Communist revolts in Germany (the fall of the Berlin wall), Poland, and the Ukraine. These revolts stir up greater change in Russia; leading to the USSR's ultimate fall.