The USSR is the largest state our planet has ever known. Managing such a state is a logistical nightmare, especially when in reality it is made up of several distinct smaller states, each of which have their own unique identity and culture.

It's a huge drain on resources to maintain central control through micro-managing the ever-changing affairs of so many smaller states.

At the same the more self-governing control you cede to smaller states, the more the smaller states develop a taste for it, decide they like it, and continually decide they want more for themselves. This is all well and good until a constituent state decides their interests are no longer aligned with the greater population, then you start to have outbreaks of nationalism. When this happens in several different states at the same time, you have a big problem. Once the tide has turned so strongly, you potentially have to use a certain amount of brute strength to keep your country together, although this is predicated upon the president's leadership being secure, which Gorbachev's certainly wasn't (he ended up having to deal with an attempted coup d'etat). Alternstively you have to respect the right of smaller states to govern themselves, which inevitably leads to domino effect of independence for one smaller state after another.

It was a gradual process, but one which progressed at an accelerating rate towards the end.