Rumor has it that the Soviet Union collapse was staged by the KGB. Is this true?
it wasn’t Reagan’s fault?
No, it is not true. USSR fell apart because of massive neglect by old rulers and their (also old) assistants. The office of the Secretary General was transferred from one old guy, who died at the job, to another, even older guy who is also about to die on the job. I don't mean to say that they actually *worked* on the job, of course. They probably never knew how to manage a country, but their age removed even those remnants of sanity.
With all that neglect in full swing, incompetence reigned on all levels and there was no [market] mechanism to remove bad managers. At the same time good managers were not allowed to work. This destroyed economy; food and other products were hard to find, with people forming lines hours before a store would open and sell maybe 10 or 20 pieces of something. Money meant nothing even if you have it; you also needed to know who has the goods to buy with that money. People who controlled goods wielded immense power and were corrupt. All that made people receptive to opposition's voices.
Role of Gorbachev should be also understood - he refused to use force to keep USSR in one piece and Poland under control. Instead he decided to allow opposition to speak, and started instituting his own reforms. Those reforms did nothing to fix the problem and only weakened the union. Regional leaders, smelling blood (and money) in the water, started declaring independence and leaving the USSR. Yeltsin, leader of Russia, did the same. Pretty soon Gorbachev found himself a president of a union without members. That's when he resigned.
If you ask why member states of the USSR walked away, the simplest answer would be "greed." People who were just local governors became czars, with privileges of making laws and doing pretty much anything they wanted. Consider that those people were already power-hungry; when they saw an opportunity to become kings they took it. Some former USSR republics are now nothing but pathetic dictatorships; other are balancing on threshold of a war; a few other do more or less OK.