I completely disagree: This is DEFINITELY a reason to freak out.
The amount was too big to be a whale. Which means it's a bug.

If it's a bug, who just ate most of that massive loss selling eth at a super cheap price?

Oh... gdax did. Which means there is a very real possibility that GDAX just lost a HUGE amount of customer funds. Pull your money out now, and play it safe. If GDAX eats it and you are still in it, you wont get your money back.