So in past bubbles it was:
1. collapse of biggest exchange Mt GoX.
2. "Bans of bitcoin in China"
3. ...
What could happen now to cause price crash? I mean, exchanges are so mature now and there are a lot of them, that failure of one exchange wouldn't cause big price drop.
What do you think can cause price of bitcoin to crash?
A failure of an exchange would have a significant negative impact because it would harm everyone's trust in all of the other exchanges. Many people would think that they have profited a lot off of bitcoin, but they are now realizing that holding it is kind of risky. Unlike money in a bank, bitcoin is not insured by the FDIC so if you lose money, it's lost. Fear would cause people to sell... especially since right now, the vast majority of bitcoin investors have profited.
Any negative news could cause a crash in the right circumstances. As bitcoin rises faster, the severity of negative news required to cause a crash lowers. If bitcoin spikes too high then any small amount of bad news can trigger a snowball selling effect.