"For now the catalyst for the move appears to be a wave of selling from Coinbase HODLers as the exchange began allowing sends and receives in Bitcoin Cash (which split off from the original bitcoin on Aug. 1 after a group of developers decided to try to improve bitcoin transaction speeds and costs)."
http://www.zerohedge.com/news/2017-12-19/bitcoin-crashing
http://www.zerohedge.com/news/2017-12-19/bitcoin-crashing