NYT columnist David Brooks has a good take on how the president goes from an approval rating of around 50 percent to, at least according to some polls, the low 40s.
My own theory draws from my meta-narrative about recent American political history. In normal times there is one majority party and one minority party. After the New Deal the Democrats were the majority party. After Reagan, the Republicans were. Then we went through a period from 1996 to 2004 when the country was basically tied. Now we have two minority parties. Both parties are unpopular, while the number of disaffected voters grows. This kind of period is extremely volatile because many voters are not for anyone. They vote against whichever party they hate most at that moment. Since hatred is so common you get these big swings. You had that from 2008 to 2010 and in mini form you had that with Obama approval this month. They were busy hating the Republicans all winter and then suddenly they remembered they don't like Obama either.