While Apple just announced the news, there are already multiple reports leaking about why he's gone.
In short, Forstall's abrasive personality combined with a series of high profile errors seem to have done him in.
Jessica Lessin at The Wall Street Journal says, Forstall's departure from Apple was "not his decision." She also says he was clashing with Apple executives recently.
Forstall has long been reported to be a decisive figure inside Apple. A year ago Bloomberg BusinessWeek wrote a profile of Forstall, and our main takeaway was: Forstall is a polarizing, political, "a-hole," who has forced out a number of Apple's executives because they hated working with him. He's also a hard-working genius, and a mini-Steve Jobs.
oh oh.. the ship seems to be sinking
Btw, this guy was one of the people who thought would have a really good chance of replacing Steve Jobs due to his similar attitudes and perfectionist approach. Doesn't seem he will be leading anything right now