BTC isn't crashing, it's selling off like any other asset that has risen a large amount. Everyone that buys has a value that they are willing to sell at. When you reach that level, those people will look to sell, which will suppress the price slightly. That could trigger other people into selling ("It's not going up anymore, better take profits!"), which builds on that decline. But you are also hitting equilibrium points along the way, so other people may decide to buy after a decline. The end result is that the value falls from X to Y, where those variables are really freaking complicated to figure out and always changing.

tl;dr: This is an asset that rose from 800 to 8000 in under 12 months. The fact is that some people like profit and don't always HODL. The result is a price decline borne out of nothing but BTC investors acting rationally.

It's mind-boggling to me that I have to explain that a 1000% increase often ends in a short selloff. That's just very very basic investing there.