It is because during a mining difficulty adjustment they were flooded with fake transactions, then it was surprise attacked by coinbase/gdax who just launched bcash support out of thin air and locked off selling and bitcoin options causing a huge price increase in bcash.

This then caused bitcoin and altcoin holders that trade and do flips to sell alts and buy into bcash which is a pump and dump fork yet coinbase/gdax traders were blocked from selling which caused it to keep rising, coinbase is now currently being investigated for this market manipulation.