My understanding is that the "crash" was actually better-informed than Twitter had intended. They do have a bad earnings report, which was released sooner than they intended. Perhaps before various insiders' sell orders had a chance to complete? b^)
The major markets (NYSE, NASDAQ) require that major news be released 15 minutes before the markets open or after the markets close, to give participants time to "digest" it without racing to trade. Earnings are almost always released just after the markets close or early in the morning for this reason. E.g., here's a schedule of upcoming releases: http://www.earningswhispers.com/calendar.asp .
A major news release by the company during trading hours is supposed to trigger a 15-minute pause in trading (which we saw in Twitter's case).
It's an open question as to whether that makes sense in an era of nearly 24/7 liquidity in official and unofficial after-hours trading networks, but Twitter was likely trying to follow official protocol. It's also not clear when there's so much other near-instant trading, but the idea is that the company can at least (in theory) give a level playing field for its own news by releasing when everyone has time to read before trading.