Microsoft's official statement from CEO Steve Ballmer was obviously very neutral, but it does hint at one possible reason for the executive's departure (emphasis ours):
We’ve built an incredible foundation with new releases of Microsoft Office, Windows 8, Windows Phone 8, Microsoft Surface, Windows Server 2012 and ‘Halo 4,’ and great integration of services such as Bing, Skype and Xbox across all our products. To continue this success it is imperative that we continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings.
Thats one explanation, but AllThingsD has heard differently. According to their sources, Sinofsky was outed because there was, "growing tension between Sinofsky and other top executives." That seems to have been validated by Microsoft's statement, with the company likely desiring less of a "control freak" leader.Julie Larson-Green will take the lead on all Windows software and hardware engineering, while CMO and CFO Tami Reller will take control of the business side of Windows.
Source: Microsoft, AllThingsD