Part of the current selloff might have to do with the fact that Bitcoin Cash will have a (non-)emergency difficulty adjustment tomorrow. This will make Bitcoin Cash easier and thus more profitable to mine within relatively few blocks. There's a possibility that some of the smaller BTC-miners will start to take their business to BCH in the coming weeks.
Judging from the current sharp price increase in BCH and (slight) selloff in BTC, it would seem that people are already anticipating some thing or another.