Gorbachev pulled together a whole host of Soviet reformers, to whom Andropov had come to be sympathetic. Their ideas had been around, and at times listened to, and to some extent implemented, at the highest level for years. Never before, though, had they so directly had the ear of Soviet President, so they were able to go a lot further. The process took on its own momentum, resulting in the fracturing of the CPSU.
It really wasn't a case of one brave man standing up to the 'totalitarian' politbureau.
Wasn't it Trotsky who said that the CPSU contained, beyond the official ideology, a range of views, from far-left to far-right? The aftermath of the party's split seems to have vindicated him.