What is interesting about the current price decline is that it’s happening amid what appears to be record transaction volume. So, the bitcoin network is seeing larger amounts of activity and a dramatically falling price. (Coinbase data mirrors the price, volume divergence.) If bitcoin is more popular than ever, what is driving the price down? I have seen it speculated that bitcoin mining operations may be forced to quickly sell more of the bitcoin that they mine in order to cover costs, such as electricity. A lower price per coin means that more coins might be sold off to cover nearly fixed infrastructure costs that miners face. More selling, without more buying, leads to lower prices.

http://techcrunch.com/2015/01/14/bitcoins-price-falls-further-amid-record-transaction-volume/