The reasons why I think he left:
1) he was a bit of a tyrant with the divisions that he ran. a lot of good people left under his reign. so maybe this is Microsoft's way of stopping the talent bleed that's been happening.
2) the reason why Sinofsky rose to his level was he ran a very tight ship and demanded that deadlines be met no matter what. he came in after Vista and set the ship right with Windows 7 and 8. he finished with a serious revamp of the windows division, the office division, etc. maybe he's burned out as well.
lots of articles in the past couple of years talking about his tyrannical ways:
Meet the Next CEO of Microsoft: Steven Sinofsky Is the Heir Apparent
excerpts (go to link for full text):
He's also an extremely polarizing figure. Stubborn. Secretive. Dictatorial.
Several people we spoke with for this article claim Sinofsky's influence and personality drove them out of the company. Another former employee called him a "cancer." Others used much ruder words than that. But even his biggest detractors admit he's brilliant when it comes to shipping complicated, high-quality software on a regular, predictable schedule.
Eventually, the word trickled down: One former manager in the online group told us that he was required to read and understand Sinofsky's internal blog posts, and his team was expected to do things Sinofsky's way.
Several former Microsoft people claim that Sinofsky's growing influence is a big reason many senior execs and engineers have left in the last three years - not because Steve Ballmer fired them for poor performance, and not because they disagreed over fine points of strategy.
One said that computer engineers view coding as both an art and a science. A lot of them were pissed off at having somebody else dictate their art to them.
Steven Sinofsky: Microsoft's controversial Mr. Windows 8 | Microsoft - CNET News
excerpts (refer to link for full text):
Two years ago, Microsoft Chief Software Architect Ray Ozzie was working on a file synchronization technology that would make stashing and grabbing pictures, documents, and music from any device a cinch.
Ozzie, hand-picked by Microsoft co-founder and Chairman Bill Gates to succeed him as the corporate visionary, gathered a team of 50 or so employees to piece together the concept. Something of an industry legend, Ozzie was a co-creator of Lotus Notes, and he joined Microsoft in 2005 after the company bought his startup, Groove Networks. To hear Microsoft insiders tell it, Ozzie was stepping straight into one of the epic turf battles that have come to define the company over the years.
Ozzie's vision -- part of an offering known as Windows Live Mesh -- became a threat to a different concept championed by Windows President Steven Sinofsky. His team was working on a similar feature for its SkyDrive Web storage service. The dispute, recounted by four current and former Microsoft executives, centered on Sinofsky's objection to the development of a service that ultimately might become part of Windows that he didn't control. Sinofsky took the fight to Microsoft Chief Executive Steve Ballmer, arguing that relying on development of the service outside the Windows division could delay the next version of the operating system and undermine the company's flagship product.
"Steve [Ballmer] made the decision to fold Windows Live Mesh into Steven [Sinofsky]'s organization, and Ray fought that pretty hard," a former executive said. Ozzie announced plans to leave Microsoft in October 2010, shortly after the decision.
But Sinofsky's critics say he's elevated those battles to a new level, thriving by marginalizing rivals while running the company's most profitable businesses, Windows and Office. Along the way, he's created a rigid product development process that puts more control in his hands and, those critics say, diminish the ability to innovate at Microsoft.
The Sinofsky Way
"You are told what to do now," said a current Microsoft executive. "It puts more directional control in the hands of the leadership."
One former senior executive referred to the approach as "Soviet central-planning." In a blog post quoted in "One Strategy," Sinofsky acknowledged that the organizational model is "controversial" but said it was a necessary structure to ensure workers could focus on their areas of expertise to meet product planning goals.
Sinofsky has also made plain his distaste for skunkwork operations, where small teams are created outside of organizational structures to gin up entirely new concepts. Microsoft created just such an team in the late 1990s to work on video gaming technology that ultimately became its successful Xbox entertainment franchise, which generated $1.3 billion in operating income on $8.9 billion in revenue in the fiscal year that ended June 30. Microsoft also set up Pioneer Studios, a group that worked on
an ill-fated tablet device that might have debuted about the same time as Apple's original iPad had it not run into staunch opposition from Sinofsky.
What's more, many of the would-be rivals for the job, senior executives who have also run the biggest Microsoft groups, have left the company in recent years. Jeff Raikes, who ran the Office group and Microsoft's sales operation, is now
chief executive of the Bill & Melinda Gates Foundation.
Robbie Bach, who led Microsoft's entertainment division, retired. Kevin Johnson, who led the Windows division and Microsoft's sales force, is now
chief executive of Juniper Networks. And J Allard, a younger leader who launched the Xbox business and led the skunkworks tablet concept Sinofsky opposed,
left the company in 2010.