Mt. Gox.
Will it happen again?
Mt. Gox had 75% of trading volume I think.
So if it happens again it cant be an exchange. Has to be
something else which is centralized. The other idea would be the failure of USDT, but I think perhaps dependence on it has waned since earlier in the year? The other idea would be a massive crackdown on ICOs, but I doubt this will come all at once.
ICO crackdown would hit on ETH mostly, I think the initial Ethereum ICO has been the last big one done in BTC. Not a systemical risk.