Everyone has its aim price especially those who trade regularly, there's someone who will sell no matter what's the situation is. For those who panic sell, sometimes its related to the current news, on what we can see on twitter, socmed and frauds. But I see no reason why would someone will think btc price will go to zero, not unless of you're craig.
I think it also often happens because they invested more than they are willing or can afford to lose. You are trying to recoup as much as you can and conduct a type of damage control. The real harm is done when you panic sell because of weak market signals and the market recovers shortly after.