You buy because you are expecting good news to be coming. Bad news comes instead. Your whole reason for being there is now gone because whatever good news comes isn't going to trump the bad news that is here now. You sell.

Or you buy because there is a crazy amount of hype on the stock and a history of insane growth. Your expectation is more of the same. That suddenly changes dramatically. You sell.

The stock has been traded mostly on pure momentum and speculation of late. That kind of scenario is highly sensitive to any kind of news.

I'm surprised the tumble wasn't worse than it was. There must be some real solid long term money in play.