That's because short positions are defined by using borrowed money. Not only will they lose money in a crash, they will be bankrupted into unrepayable debt by a crash. So if they think it's crashing, they have to sell before a certain price because they do not have the money to cover their position.
That's how a crash happens isn't it? It's not driven by any wider economic reality beyond the conviction of "investors". If the investors can be convinced that it's crashing then it crashes. That certainly appears to be how manipulative exploitation of short positions occurs.