The site reports that the shakeup follows several troubling occurrences at the company including missing analyst expectations for two consecutive quarters, Browett trying to nickel-and-dime employees, Siri shortcomings, a widely criticized 'genius' ad campaign, and a Maps application that was released before it was ready.
Apparently, Forstall refused to sign the letter apologizing for Maps.
I also heard that Forstall refused to sign the letter apologizing for the mapping fiasco, sealing his fate at Apple. (He has worked for two companies in his career, both founded by Steve Jobs: Next and Apple. He's also a big San Francisco Giants fan. Win some, lose some.) Seeing as Forstall oversaw and publicly demoed Siri and maps, we know at least that the Apple culture is intact: Forstall was the directly responsible individual, or DRI, on Siri and maps. Now he is gone. He will remain an advisor to Cook until "next year," Apple said in a post-market news release. This is a formality intended to keep Forstall from ringing up Samsung.
In addition it's said that Forstall didn't get along with Jonathan Ive, Apple's key product designer. Now Forstall is on the way out and Ive is put in charge of Human Interface (HI) for the company.