Sinofsky has been with Microsoft for a very long time. He joined the company all the way back in 1989, and has, since then, been responsible for several important releases, including Office 2007, Windows 7, and, of course, Windows 8. His track record is impressive; all these three were major releases, and two of them - Office 2007 and Windows 8 - were radical departures from previous versions. In other words, Sinofsky managed to make disruptive changes in a company as large and established as Microsoft, and that's a rare quality.
I'm guessing this rare quality didn't always make him friends, as sources indicate that his departure resulted from his attitude supposedly being aggressive - focussing on Windows and letting everything else kind of fall by the wayside. Now that Microsoft has clearly shifted towards the integration of its various product lines, the need for someone who can make disruptive changes to existing produces - collateral damage be damned - diminished.
Sinofsky himself of course denies these rumours in his letter to Microsoft employees, published by Paul Thurrott. "Some might notice a bit of chatter speculating about this decision or timing. I can assure you that none could be true as this was 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," he states, "As I've always believed in making space for new leaders as quickly as possible, this announcement is effective immediately and I will assist however needed with the transition."
Sinofsky's responsibilities will be taken over by both Julie Larson-Green, who will be responsible for Windows software and hardware engineering, and Tami Reller, who will take care of the business side of Windows, together with her existing roles as chief financial officer and chief marketing officer. Both Larson-Green and Reller have a long history in the industry, and especially Julie has been involved with Windows as a regular public face for a few years now.
"Leading Windows engineering is an incredible challenge and opportunity, and as I looked at the technical and business skills required to continue our Windows trajectory - great communication skills, a proven ability to work across product groups, strong design, deep technical expertise, and a history of anticipating and meeting customer needs - it was clear to me that Julie is the best possible person for this job, and I'm excited to have her in this role," Ballmer said in the press release.
I can only go by Larson-Green's public appearances demoing new Windows versions, but it seems that she might be the right choice for this one. Sinofsky always came across as very forward and overly enthusiastic - which is great if you need to shake up a sleeping company division, but not once everyone's awake and cooperation across divisions is needed. Larson-Green, on the other hand, came across as more reserved, expressing the enthusiasm for the work that she does in more subtle ways. A more reserved, careful person might be exactly what's needed to get Microsoft's divisions to work together properly.
All in all, a big shake-up, which, according to Microsoft, has nothing to do with Windows 8's performance so far - which makes sense, since it's far too early to say anything meaningful about such a drastic release.