OK, so what caused Barack Obama to change his mind and actively press for repeal of DADT this year? Offhand, I'd say there are three leading contenders:
- Nothing changed his mind. His plan all along has been to do it this summer or next, and he was willing to stay quiet and accept the hostility of advocacy groups before this because he knew that a bit of discretion on his part offered the best chance for enacting permanent change with broad public support. This is more or less Mark Kleiman's view.
- Strong pressure from the gay community forced his hand. In other words, this is an example of that old FDR legend where he wants to do something, but tells his supporters they have to "go out and make me do it." (It's worth noting that this apocryphal story1 is pretty popular among liberals as a story, but we all sure hate it in practice.)
- Obama (and congressional Democrats) are afraid they're going to lose their majority in November and will then lose their chance to push this through for good.
Because I'm a milquetoast centrist sellout, I'm going to punt and say that the answer is all three. Obama really has planned to do it all along during his first term but without a more specific timetable; the pressure from gay and lefty advocacy groups helped push congressional leaders into action and they in turn pressured Obama; and there was probably some additional political calculus related to the possibility of Democrats losing their House majority in November. That's my guess, anyway.
1At least, I assume it's apocryphal. I managed to find a reference to it from I.F. Stone in 1969 once, but I've never been able to track it back any further than that.