According to the WSJ, Scott Forstall was asked to leave Apple after he "refused to sign his name to a letter apologising for shortcomings" in Apple Maps. Forstall had wanted to address the situation of Apple Maps without apologising to the public, like Apple has done in the past. Tim Cook, however, felt an apology was needed and now Forstall is out at Apple.
Forstall, who had been at Apple for 15 years, was in charge of all things iOS at Apple and thus directly responsible for recent disappointing products like Apple Maps and Siri. There have been reports of him clashing with other Apple executives because he was notoriously difficult to work with and "never fit into the culture of Apple," according to a WSJ source. The Apple Maps fiasco and unapology was likely the final straw.
If you remember, the apology (which was eventually signed by Tim Cook) was unique for Apple in that it actually suggested competing products for users to download. Seeing Forstall, who was a Steve Jobs protege, get pushed out by Cook, because he wouldn't take ownership of screwing Apple Maps up, reveals a tiny glimpse of how Apple is now run under Tim Cook's steady hand. Things done changed. [WSJ]