They dump because of panic selling and also not seeing any potential anymore is the system. One of the main factor is the mainstream media who actually disorients the people on what the cryptosystem really is. Anothet thing is that it also becomes a platform spreading fud causing the holder to react.
They get affected by the market actions which are bearish at the moment. They take decissions quickly and they can't foresee the possible results of their decissions. That's a fact makes people dump their bitcoins.