It certainly isn't about the success or otherwise of Windows 8. Steve Ballmer's comments about Surface have been widely misinterpreted; he called Microsoft's distribution plans modest, not initial sales and UK retailers have reported Windows 8 laptop sales 25% higher than anticipated.
No version of Windows can immediately reverse the flat or declining sales of PCs in mature and saturated markets but what Windows 8 and RT do is open up to Microsoft the tablet market that has so far been dominated by Apple.
It's not about disappointment in his achievements.
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In the past Ballmer has relied on Sinofsky, first to investigate internally what led to customers and partners suing Microsoft over Windows Vista Basic (after the Windows team reneged on commitments to OEMs not to support less capable Intel graphics chipsets) and then to rescue the Windows division from the morass of Longhorn.
Longhorn OS that was abandoned after years of development and almost entirely rewritten for release as Windows Vista.
Sinofsky brought to Windows what he delivered in Office; an organization that shipped high quality releases on time.
Equally, this isn't the result of a failed attempt to become Microsoft CEO. For one thing, Ballmer (and more importantly the Microsoft board that actually appoints the CEO) isn't looking for a successor; in 2008 he said he planned to stay at Microsoft for another decade.
Nor is it some belated revenge for ousting potential CEO aspirants like former Xbox head Robbie Bach.
While Sinofsky may have helped kill off Bach's plans for a non-Windows tablet, Bill Gates' desire to see Courier do email was just as lethal.
Creating another platform at Microsoft just as Xbox and Windows Phone finally became part of a single Microsoft platform alongside Windows and Windows Server was a terrible idea and Bach's departure was far more about dismantling the internal fiefdoms that had become rampant at Microsoft at the end of Bill Gates' tenure.
Timing "not significant"
If anything, it seems more like the departure of former Windows Server and Tools president Bob Muglia (who left in large part because he didn't think Microsoft's cloud products were mature enough to be "all in" on).
Sinofsky himself frames it as "a personal and private choice that in no way reflects any speculation or theories one might read - about me, opportunity, the company or its leadership" and says that the timing isn't significant.
That would make this week simply the first opportunity to make the announcement without distracting from the Windows 8 and Surface launch or the Build conference (where he was noticeably absent, unlike the developer conference right after Windows 7 shipped).
It's not impossible that after 23 years at Microsoft and two marathon rounds of Windows development cycles, he simply wanted a change. Or, to indulge in the "chatter speculating about this decision" that Sinofsky predicted his departure would cause, it might be about internal disagreements about ways to run the various Microsoft divisions.
Although Bill Gates is still heavily involved in Microsoft and in regular email contact with many senior leaders at Microsoft, Ballmer seems to be putting more of his own stamp on the corporation.
He has clear ideas about the way divisions should be organised and run and if Sinofsky disagreed about that (as other Microsoft division chiefs have in the past) or wanted to work in a different way, it would mean a parting of the ways.