People dump their bitcoin in fear of bitcoin dropping again. Unbelievers.
It's also because some people bought bitcoin for about $200, and since it's gone up, people are running with the little profit they made.
And since it dropped on the weekend to around $230, it gives people a stronger reason to sell and run.
 Anf second reason is about  panic
Yes, panic is always the main reason of all dramatical drops, not only bitcoin-related. The people that sell their coins out of panic not only lose their own money but also make others lose theirs.