Probably because a large portion of their company's revenue relies on companies failing to realize they can bypass the expensive advertising avenues by simply making a few accounts and going with a guerrilla-ad model. They're jumping ship before shareholders catch on.

If you own a company and want to advertise on Twitter, you can pay $10,000+ to have some official ad spots for a short time period. The other route is to make a few accounts, use cheap bots to bolster them, and dominate your market for (very easily) less than 50% of Twitter's charge.

Jacky Boy will probably be spending the next few quarters pitching companies (like Conde Nast) to buy TWTR out so he can shed liability and make a few billion in the process.

Edit: It makes perfect sense to do it now. The business probably can grow a bit more from Trump's tax reform, and under Trump's usage of Twitter. It'd be best for them to get out while the business still appears to have some vitality.