all the time before August 1st (before the Jul 15-20 approximately) there was a fear of the fork. not the Bitcoin Cash altcoin fork but the SegWit fork because it never had any support and the UASF proposal was pushing for a split. and on the other hand bitmain was also pushing for another split and there was the legacy chain. things would have been messy. obviously every investor thinks twice about the future and gets their money out, even if it is only a part of it not all.
but as SegWit2x locked in the BIP91 all those people gained their confidence back and started to come back. as it was a 100% support, they came back with 100%+ more money this time. hence the big rise.