Twitter has a rather spotty record regarding security. There are serious discussions in the trade press suggesting that if Twitter does not act soon to address both the substance and the perception regarding Twitter security, the marketplace may punish them. I agree.
Apparently from automated trading systems that immediately react to headlines monitored by software. It's being reported that the SEC (and perhaps the FBI?) is looking into who profited during the 3-5 minute period that the market plummeted then rebounded.
These automated trading systems do more harm than good, IMO.