There are different theories.  The best two I've read are:

1) Margin buyers got caught and had to dump creating a big selloff which caused more margin accounts to be forced to sell on exchanges like Bitfinex.

2) This was manipulation cause 2 days prior swaps on Bitfinex shot up meaning someone shirted a ton of BTC which may indicate prior knowledge or a plan to crash the price.

Given it's summer and volumes are thin it didn't take much to crash the price.

The question now is:  How low will it go?  I think $400 although a very brief $350 may not be out of the question.  I would be a big buyer here if I had the money cause this thing should go crazy the 2nd half of this year.

Cheers!