There's a what, another what, and a third what, but no real "why". Yes, the lockup period ended, but plenty of company's stock prices have been just fine after lockup periods expired.
Once the lockup period expires many people have to sell a ton of shares to cover the taxes they have to pay on all the shares they just received. Say you make a few million on Twitter stock because you joined 3 years ago. Unless you've got a million bucks cash sitting around you're gonna have to sell some shares to afford the taxes. The combination of everyone doing this at the same time is the "why".