It is really hard to tell.
I heard sometimes, 2015 will be the year, the US-government will try to destroy Bitcoin. Maybe it is that.
There was the explanation that, it goes down, in December because people are selling BTC to buy gifts, but that is over now.
The explanation that still makes the most sense for me is: There are some speculators, who make a lot of money with a falling price and it is their doing.