He's retiring (third Democrat from a at-risk district). It's a D+0 seat: contra the New York Times, Cook had it as being competitive (Likely Democratic), and that will be almost certainly be upgraded rather quickly. Baird's reasoning? He wants to spend more time with his family. Judging from the reactions of his colleagues, this sudden desire comes as a bit of a surprise.
Interestingly, the six-term, 53 year old Congressman was not on the NRCC's target-for-retirement list. He had three GOP challengers:
...and that's all I know about them. Well, that and that one of them just got a much improved chance to be the next Congressman from WA-03.