There are so many reasons behind it I guess people are selling their bitcoins because the minimum profit of their investment is now fully achieve and they must sell it in order to have a profit. But if they are smart enough well they will hold it as long as possible.
All the time, the same scenario. They buy it and they can't hold it. They sell it on the market for nothing, whenever dump happens, they sell their coins with cheap prices because they're not strong people.