There are three distinct phases to collapse of the USSR, and each in turn was a large contributor to the collapse, even if unintentionally.

The first phase was the slow dismantling of the Warsaw Pact and the emergence of glasnost as a strong force near the end of the 1980s. This new openness meant not only was more debate and dissent in both the USSR and sattelite countries going on, but the Red Army would no longer be protecting the communist governments in the Warsaw Pact. With the fall of communism around the Eastern Bloc in 1989, democratic, anti-communist fervor was also spreading to the USSR.

The second phase was the initial separation of those states that had no interest in remaining in the USSR, particularly the Baltic and Caucasian republics. This occurred in 1990, thanks to the communists losing the elections in those republics. While not every one of these republics declared independence immediately, they did begin the formation of national institutions independent of the Kremlin, beginning the transition to independent states.

Now, at this point, it is possible the USSR or at least a successor federal state could have been salvaged. Belarus, Ukraine, and the Central Asian republics could possibly have been saved into a new treaty that would have saved much of the territory of the former Soviet Union, although the successor state would not have been the USSR most likely. Indeed, although several states were breaking away throughout 1990, by 1991 things were settling down and negotiations were underway to move the republics of the USSR forward into a new system.

The final pin to drop was in August of 1991 during the coup. This completely eviscerated trust in the Soviet institutions that would be transformed into new federal apparatuses. The response in the USSR was swift, but most accutely felt in Russia. The coup itself was directed mostly against the new government of the Russian Republic. This meant that the largest republic in the USSR was now directly opposed to the continued existence (or transformation) of the USSR. Without Russia, the USSR's collapse was swift. The actions of some of the leadership of the Soviet Union's institutions meant that the newly democratic republics had no interest in saving it.

A combination of factors led to the dissolution of the Soviet Union. I will detail some of them here in the hopes of answering your question.

1. Leadership crisis of the 1980's.

Leonid Brezhnev ruled the Soviet Union from 1964 until 1982. The last several years of his rule, he was infirm and routinely absent when key decisions were made. Shortly after his resignation and death, Yuri Andropov became General Secretary. Already a senior citizen and in poor health, Andropov held the post until his death some 15 months later. And so the torch was passed to Konstantin Chernenko, already elderly and infirm, who carried it further another 13 months until his passing on 10 March 1985. That same month saw the ascension of Mikhail Gorbachev, a younger man by comparison, who had just turned 55 years old.

2. Gorbachev's Domestic Reforms.

Principally Glasnost (openness), Democratizatsiya (Democratization) and Perestroika (Restructuring). This package of reform sought to restructure the economy (perestroika), enable a widening of freedoms, including freedom of speech and the ability to criticize the government (glasnost) and bring about competitive democratic elections for lower ranking party posts (democratizatsiya). These reforms were resisted in various ways by the constituent republics of the Soviet Union, and created factionalism within the Communist Party itself. There were hard liners looking to end and roll back reforms and others who wished to see them accelerated. Another part of the Glasnost package was to allow foreign broadcasts into the Soviet Union which had previously been jammed to prevent citizens from tuning in to foreign ideas. Finally, Gorbachev abandoned the Brezhnev doctrine of intervening in the internal affairs of satellite states to ensure Soviet hegemony within the Eastern bloc. Gorbachev demonstrated his repudiation of the doctrine by not intervening when Poland held free elections in 1989, and the Communist Party there was defeated by Solidarity. Other satellite countries and even member republics got the message and began to push forward their own separatist agendas.

3. Economic stagnation and rising expectations.

The military budget for the Soviet Union was significant enough to cause a shortage of industrial goods for Russian consumers. With a more open media environment and freedom of speech, the demands of the Soviet citizenry for a higher standard of living began to creep into the public sphere. Agricultural setbacks and the need to import grain coincided with a decline in oil prices, forcing the USSR to borrow and import to prevent food shortages in major cities. This limited their ability to project force and may have been the principle reason behind abandoning the aforementioned Brezhnev doctrine.

To be sure there were other reasons that the Soviet Union collapsed. Those are just a few and I hope they help.

I read "Armageddon Averted: The Soviet Collapse, 1970-2000: Soviet Collapse Since 1970" by Stephen Kotikin a few months ago and, although it felt like a shorter work that had been padded out, it had an interesting take on the Soviet collapse. In short Gorbachev misunderstood what held the USSR together. He thought that the non-political structures of army, secret police, civil service and economic management bureaucracy could hold everything together and that the Communist Party itself could be removed and allow a democratic USSR to continue. In fact the Party was the only real power that covered the whole USSR and once he attacked and weakened it there was nothing to stop each republic from going its own way. The book tried to explain why the breakup was so relatively peaceful rather than the fundamental causes of collapse, but he does mention the 1970s oil price spike as covering up economic failure. Once the price of oil fell the underlining economic mismanagement was revealed.

I read during the collaspe of the Soviet Union in Russia from 1990to 2000s it was hell for those people because of corruption, murders, petty crimes, gang violence, rape, and chaos roamed the entire country. Not all parts of former USSR because some places usually the urban areas are at chaos farmland were probably at peace when the new political parties emerge.

Remember Boris Yeltsin once in power decided to attack a Communist building because like I said it was rough years where Communist party still hold huge amount of influence.

He did it out of fear which made him a target in Russia and fled the country because Vladimir Putin was also supporting the Communist party remember he was made by them.

EDIT

I'm going to be reading more about the collaspe of USSR