Don't think selling nothing but only a panic is the reason because many traders might have brought at lower level prices so they know that these prices may go down some more that's why to make profits out of that they will sell and buy back again when prices to some more down. This is purely a trading strategy so most common people may sell in panic but traders will take that advantage.
This usually occurs due to the operation of the market .. People are afraid to drop prices. But no one said anything about the most obvious thing - People just need the money!