Lot's of people made lot's of money in the last couple of month and taking winnings. Remember it was <1000 a few month ago. When people more then doubled their money in a few month it doesn't take much for them to decide to take some of that.
Additional fuel by speculators hoping to buy in back later at a lower price.

And there is a lot of uncertainty currently in Bitcoin due to the Blocksize debate and the possible split.