All of these happened in the wake of major crises.

Apartheit in South Africa ended, because of massive protests in the country (including violence), massive international pressure (the UN labelled apartheit as crime against humanity and lots of countries banned imports from South Africa), and the South African economy was collapsing due the price of gold dropping a lot.

East Germany collapsed because of massive economic problems. Storming the border crossings was just the last push. If that would have happened 10 years earlier, the police would just have shot the first row of people trying to cross the border and the rest would have fled. The DDR was already collapsing at that point. And the press conference that caused the storm on the border crossing was actually about East Germany opening the border in a month's time. So even without the storm on the border crossings, the same thing would have happend, just a month later.

The Sovjet Union, again, collapsed due to economic problems. Their economiy completely collapsed and with it their power over all the different SSRs. Russia, being the largest and most powerful SSR, being able declaring itself independent of all these other countries that really didn't want to be part of the USSR without having to use violence is really not surprising.

It's kinda as if Great Brittain declared itself independent of the Brittish Empire.

All these situations you mentioned where already under way for a decade or so, before the events you mentioned did the last push. And all of them where only possible to massive crises.