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.
For my opinion, people panic sell due to reasons. They are newbies, people tend to panic if they saw their funds losing and without further research they will sell. Fud is the next thing that tends for the other holder to sell. For their own reason, funds or an accident occurs and they need the money. Last, some people just go with the trend and without further knowledge about crypto, most of the time they are getting scam.