There is much speculation about the possible reasons for the sell-off - and as almost always, there is no clear cause.
Rather, a mix of different factors could have accelerated the sale.
Very controversial hard-fork at Bitcoin Cash causes a lot of uncertainty. At least the early crash at Bitcoin Cash makes this thesis seem quite plausible.
However, it is unlikely that an announced hard-fork can pull the entire market down. What is unusual is the fact that the sell-off on the crypto market coincided with the yield collapse in the US bond market. Most likely, there were also many stops near the October low at Bitcoin