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.
I don't buy that.
There aren't two ways to make the prices fall: Sell a lot at a low price.
That means you need to own a lot of bitcoins to start with, and dump them at a low price.
Now what's the interest of doing that? Hoping that the price will crash much much lower than the selling price so buy rebuy low. It's a hell of a bet.
If the market swallows the orders (and it did swallow $200,000,000 worth of orders yesterday) without starting a crash, the plan fails. To succeed, such a plan needs to not find a resistance in the market. Usually, there are some. So that's extremely risky. And since all of this cannot be forseeable in details, people doing that would need to keep many Bitcoins in reverve in case the pump needs to be restarted again. That means they need to be losing money with the rest of us.