Simply more supply then demand, it could be caused by a lot of reasons.
One of them- the believe of people that the price of bitcoin will go down in the short-term so they prefer to sell in order to save in costs/to buy cheaper bitcoins.
Another thing may be a strong player that decided to sell his bitcoins after buying a large quantity just to move the market to the side that he wants.
It could be a market manipulation, and the manipulator earns the ability to buy cheap bitcoins, which is very important in the cryptocurrency market.