Up until now, the prevailing theory had been that Sinofsky was kicking out all of his competitors. My understanding is that Bob Muglia left because of direct conflict with Sinofsky and Robbie Bach seemed to be on the loosing end of a few Sinofsky scuffles.
It may be that it just caught up with him. Or maybe he got too antsy for the CEO role.
Also- this looks incredibly bad for Ballmer. Sinofsky was at Microsoft his entire career and was head of Office before Windows. You don't promote someone like that without knowing what they'll manage like.
Ballmer has lost almost all of his original group of Presidents (I believe Qi Lu is the only survivor). If the board is worth anything, they have to be real pissed at him- all potential successors have now left the company. Microsoft's politics are legendary and Sinofsky was seen as one of the few who could navigate them, now he's out.
I wouldn't be surprised if the board of directors had some extremely harsh words for Ballmer.
It may be that it just caught up with him. Or maybe he got too antsy for the CEO role.