This is a 5 mins chart of Bitfinex, i suspect that the decline may have been started by few speculators who knew that due to the margin call effect would have done the rest stacking and forcing many sale orders in multiple accounts provoking a quick flash crash that pushed the market enough low to allow them to rebuy at a much lower price.
Aniway the price is recovering and moving towards the fast VWAP at $535 that will act as resistance for the next few days.
UPDATE:
An interesting article about what supposedly happened at BitFinex.
Bitcoin Trading Ideas, Analysis 