There are at least two ways to explain it:

1. A play-by-play timeline or proximal causes. Gorbachev, economic crisis, Yeltsin, illegal dissolution + coups, Shock Therapy.

2. An anslysis of the more important and actual causes that made the proximal causes possible at all. An abandonment of class struggle, the conversion of the party to leading faction bureaucracy over political struggle and development, challenges in recovering from the decimation of their population in WWII, the slow invasion of liberalism and failures to understand propaganda (USSR academics began to believe that capitalism in the USSR would actually benefit them like it did imperialists!), unpopular economic planning decisions (e.g. insufficient consumer goods), and of course, constant and extreme outside pressure from imperialists (sanction, sabotage, wars, brinksmanship, propaganda), and not enough friends to support each other through challenges (I believe that without the Sino-Soviet split, the USSR would still exist).

I have in no way presented exhaustive lists and will have lefy out important things. You won’t understand it without reading the types of longer form materials already presented and without knowing the real histories around the USSR. Its inception, immediate invasion by imperialists, the fallacious propaganda machine about Stalin (there are plenty of criticisms if times under Stalin without the nonsense), etc etc.