Maybe they shorted and then announced the rule changes...

That's the fun with the stock market, you can never be sure which is the cause and which is the effect. Used to be the SEC would keep an eye out for shenanigans, and occasionally they'd prosecute some fraudulent activities. But now you don't know if a stock drop was caused by an action or some shenanigans on the stock market made someone take an action to intentionally drive down something they shorted.