Obviously it's a mistake to see the post-war pre-1989 Eastern European countries as one huge homogenous monolith governed by Moscow, but each of the Eastern Bloc states did either collapse or rapidly wither away during the historically brief period from 1989 onwards. The events in each country had certain basic things in common.
1. The economies were in tatters. Call it soviet-style communism, state capitalism or whatever you like, but by the late 1980s it had ruined the economic life of the countries.
2. The Communist Party leadership was ideologically bankrupt. They could no longer claim to be marching towards freedom, equality and prosperity for all. The 40 years of spying on each other and political repression was enough proof for the citizens that they were not really marching in any direction at all.
3. People could see and hear for themselves, on foreign TV stations and on radio drifting in from abroad, that things in neither the west nor their own countries were as the leaders told them they were. Furthermore, they could keep up to date with events in Russia and in their comrade nations, and know that changes were taking place and that their own leaders were starting to panic.
4. In most countries, there was an organised opposition. It may have been in prison or in exile, but it existed and it was clear that its ambition was to take power away from the Party.
5. In a lot of countries, the organised oppostion was led by a very clear figurehead. Probably ten years before, neither the leader of Solidarity Lech Walesa nor the intellectual dissident Vaclav Havel would have imagined that they would one day become President. But as events moved on it must have become clear that the people flooding into the streets and squares saw them as leaders. Hence it is very easy to look back now and see them as Presidents-in-Waiting.
6. According to one account I once heard (repeated here), the KGB had a decisive influence on events. They had allegedly decided to oversee the removal from power of the first generation of Eastern Bloc leaders, and settled on street demonstrations as a means of achieving this. Things subsequently got out of hand - I'd imagine that Vladimir Putin was not well pleased.
Then there was the domino effect, and the most significant factor here must have been the USSR itself. Change in Russia, as we all know, was not led from below but from above. Mikhail Gorbachev was, like De Klerk in apartheid South Africa, an insider who wanted to essentially preserve the system, but realised that it would have to change if it was to survive:
The Party, which I had joined, itself badly needed to be reformed and reoriented toward democracy. And through this, the country could begin to gain some freedom. That came later, but it all started with the desire to do something and show initiative. That was what led many good people to join the Komsomol (Communist Youth League) and the Party.
At a certain point, though, the momentum for fundamental change had built up to such an extent that 'die Wende' was reached. There was no turning back.
So when in August 1991 some Communist Party hardliners briefly kidnapped Gorbachev in an abortive coup attempt, the world saw them for what they really were: desperate old men whose time had passed. Not only were they no longer at the wheel of the ship of state - they had been thrown overboard.
Incidentally, the fact that the ex-head of the KGB is firmly entrenched as leader of the Great Bear tells us a lot, I think, about the difference between overthrowing a totalitarian state through popular uprising, and waiting on the leadership to quietly reform it and their own positions, powers and privileges out of existence.