If I noticed anything from last night's debate it is that it seems to be that Obama almost seemed like the challenger. It could be that he might have felt that a more aggressive performance was what he needed to best Romney in the debate. However, I think there is a simpler reason: a tough Obama livens up his base. He's busy fundraising with rappers and he's hanging out at a ton of college campuses in the last two weeks of the election which begs the question; why isn't he making speeches to independents? Simply put, he must feel from his internals that his base isn't yet where he wants them to be to win the election and knows that without maximum support from these core constituencies he'll come up short.
It seems, for the most part, that Obama has given up on whole segments of political independents, especially white indpendents (right now he's getting less than 40% of the white vote, which is very dangerous for a Democrat in a national election), so he's going to spend most of the rest of his time trying to squeeze out every last vote from his base. It is hard to imagine him getting the support from African Americans and college-age voters that he did in 2008 but he'll need them all to win. If he doesn't get that massive turnout from these groups he'll probably lose the election. That explains his aggressiveness and the last two weeks of his campaign schedule.