There is a rumour that the drop is linked to the goings-on in Kazakhstan.

Apparently a lot of bitcoin miners moved there because electricity is cheap there. Mobs make anyone with assets nervous. Then the internet went down in Kazakhstan last night interfering with mining. Any miner with a satellite phone out there would have connected to an exchange and sold btc for dollars with a view to fleeing.

If the drop is Kazakhstan-related, we've probably seen all the selling we were going to see, last night.