It seems as if Sinofsky has simply decided that it’s his time to go, after helping to ship various successful products, culminating in the launch of Windows 8 and the Surface RT. Ballmer, Microsoft’s CEO, had this to say in a letter to the Windows team:
As we enter this new era, and with the successful launch of Windows 8 and Surface behind us, Steven Sinofsky has decided to leave the company. Steven joined Microsoft in 1989 as a software development engineer and has contributed to the company in many ways from his work as a technical advisor to Bill Gates, to leading the evolution of the Microsoft Office business, to his direction and successful leadership of Windows and Windows Live as well as Surface.
Sinofsky has been with Microsoft since 1989. Sinofsky rose to power with Windows 7, and recently was the major driving force behind Windows 8 and the Metro interface, as well as Microsoft’s Surface RT play. Many considered him to be the obvious successor to CEO Steve Ballmer.