There are two major reasons that makes the bitcoin value drop which are:
1. Panic by bitcoin holders - When there is negative news about bitcoin the panic people are the first to sell. And if there's is too much dump in the market that will drop the bitcoin price.
2. Speculations and FUDs - Speculating fake or negative FUDs usually makes the value of bitcoin drop because a lot of people are going to be scared of what is going to happen.