Probably leaving the path open for Pelosi to be succeeded by Xavier Becerra as soon as Hoyer retires, since Clyburn doesn't look likely to go for it.
So, why would Van Hollen do this? Is it that his House career looks like a dead end? Pure boredom, like with Markey? Or ambitions for an office that is easier to aspire to from the Senate than the House?
Unless you're speaker or minority/majority leader, being a senator is always an upgrade to being a congressman.
Van Hollen has devoted his life, since 2003, to Democratic congressional leadership, and it is very likely that when Pelosi and Hoyer step aside either him or Xavier Becerra, of California, will become Speaker. I'm inclined to think he has more power in the House than the Senate, but he might be anticipating it being removed. My tweet was oblique future presidential speculation.
There is, I realized the other day, another way for O'Malley to stay relevant after 2016 if his presidential run doesn't get him notoriety or an appointment: Maryland only has a consecutive gubernatorial term limit, so he could be the candidate against Hogan in 2018.