Market sentiment.
When see everything drop in price, you also fear yours one also drop, so some may want to sell first or at least trim the holding to minimise the risk of dropping.

You don't need every company shareholders to sell to cause share price crash. Just more sell > buy, then price will go down.

Once dropped too deep, those under margin account, need to top up, if not, investment bank will sell on behalf, if it did, then it compounded the crash.
But once those forced selling ended, market stablise and up again.