I don't think it has anything to do with Bitcoin Cash . It is a manipulative dump which has triggered a lot of shorts. The speed indicates it is mostly an electronic move (ie sell orders being executed automatically as price points are hit). It has dropped down just a tad below the Sep 2017 pre-boom low; so if it can recover over the next couple of days up to the previous sideways trading range and this becomes a false breakout that would then actually be a very bullish sign. If it busts though 5200 levels and convincingly breaks down then it is anyones guess how low it goes before it hits bottom. A real breakout needs time not just price to go with it.

Think about it ; if you had a few million in bitcoin after weeks of sideways trading with tightening volumes you could just dump on the market and sell, cause panic and trigger everyone's electronic sell orders , then just buy them back 4 hours later at a 15% discount. This kind of manipulation is exactly why the ETF's for bitcoin keep getting rejected.