Someone decided to sell around 100k ETH on GDAX in a single order and whiped the whole sell book.
Like a certain mr102 did in btc-e some years ago, but now with steroids
Probably he started to panicking, else he would sell it slowly in multiple waves so he does not disturb market balance. This way you can conclude this things:
1.He knows something and he was in hurry to cash-out.
2.Those are maybe stolen funds..dumped.
3.Someone predicted that this was a peak, and will start free-fall.
4.I just can't imagine that this was lonely try to cause a dump..Maybe?