Was the fall of the Soviet union a bottom-up or top-down movement?

I've read in other posts that it was the break-up is ultimately being attributed to Gorbachev's reforms. In my understanding his reforms allowed for the break-up to be possible, which nationalists took advantage of.

However I don't quite get how exactly this went down. Was there some kind of grassroots movement that took over? And if so, how did this happen so fast and suddenly if the citizens were politically very passive due to the previous decades of repression and censorship?