Reportedly, it was his refusal to sign a letter of apology for the FUBAR that is Apple Maps.
Apple CEO Tim Cook issued an open apology letter for Apple's new Maps app, which has been called an incomplete product by some and dangerous by others.
That letter was the one that Apple wanted Forstall to sign.
Certainly, the lesser debacle that is Siri also hurt Forstall's reputation at the company. Also an incomplete product, Apple's voice assistant frequently returns incorrect results or simply doesn't work.
Forstall is a 15-year Apple veteran, and was a protege of Apple's former CEO, the late Steve Jobs. For years, other Apple executives had complained that Forstall was abrasive and flaunted his close relationship with Jobs.
That relationship did not lead to Forstall replacing Jobs, in the end, as Cook instead was elevated to the CEO position once Jobs retired due to health reasons. Since then, sources said, Forstall has continued to clash with other executives.
In an unrelated move, Apple also axed its new retail chief, John Browett. Browett only joined Apple a few months ago, but made some mistakes that drew criticism, not just internally, but publicly. Included in those errors were a decision that cut retail staff hours too severely.