I think the primary or the most speculated reason I understand is because of rumours that bitcoin prices are controlled by bitcoin whales, according to Bloomberg, is about 1000 people, that hold about 40% of the bitcoins.
One such whale they say is now defunct Mt. Gox exchange's attorney called Nobuaki Kobayashi, has said, he is raising money to pay off Mt. Gox creditors.
It is also said that he holds about $2 billion worth BTCs and out of this, already $400 million dollars of BTC & BCH combined together has been sold, which is setting off panic in the market already.
RE: Bitcoin drops by over $1000 in the last hour