Bayh was a big time perpetrator of the current bullshit. On the other hand, he was basically Senator-for-life in Indiana if he wanted to be, and ultimately better than having a Republican in the seat, even if it's a "moderate" Republican like Lugar.
Unless there's a scandal brewing, the only thing I can think is that he's realized -- after years of being The Next Big Thing -- that the moment of DLC-style Democrats is over, he's not going to be president or VP, and that the current White House doesn't share his perspective and will only placate him so far as it needs his vote. Bayh's used to being treated as a lot more important than he is, and never seemed to care a ton about accomplishing actual policy goals.
Basically, he might just be a smarter, Hoosier version of Palin.