Mania and volume.

 Exchanges have come along way, but they still aren't as robust as actual trade systems like the NYSE.

We've seen explosive growth (earlier today Coinbase was reporting 7x the traffic compared to their previous all time high), and now there was a bit of a dip.

The moment there is a dip during a mania period you get the market equivalent to a run on the banks (https://en.wikipedia.org/wiki/Bank_run). People rush to sell/get money out and cause the exchange to be overloaded.

When an exchange is overloaded, more people freak out and try to sell.

Meanwhile, algorithm traders are automatically reacting to the markets and people's "Stop Loss" sales are triggering causing the price to drop down further.

All in all this results in a drawback/correction, but, sometimes you see a larger 'crash.'