My guess would be that he got an offer for a well paid cushy gig at a think tank, a lobbying firm or an "outside" political group.
But who knows with that idiot.
If he got a good offer from a place that wants him that much, they could afford to wait until January 2019 at this point, barely 21 months out.
He's too young to have a significant health issue (things like prostate cancer can strike that young, but many politicians live with manageable cancer, including at least three governors who have prostate cancer right now: Wolf-PA, Dayton-MN, and Brown-CA), there might be family stuff going on but i think he'd say so if it were family stuff, is a way of gaining sympathy.
No, something's up.
Edit: I don't think Warren will run. She's right where she needs to be. Biden seems to be floating something, and i think Kamala Harris, Gillibrand, and Cuomo are clearly jockeying already. The Castros are probably going to try knocking over Ted Cruz first, though that wouldn't preclude a run in 2020...