In an utterly surprise move Monday, Steven Sinofsky, the guy in charge of Windows, left the company a few weeks after the Windows 8 and Surface tablet launch.

His job will be split between Julie Larson-Green, who will run Windows software and hardware engineering, and CFO Tami Rellerm who will run the Windows business, taking over the business and marketing strategy for Windows, Surface and partner devices.

No reason was given for Sinofsky’s departure.

The Wall Street Journal said, “Mr. Sinofsky’s sudden departure is likely to raise questions about the success of Windows 8 and about his relationship with Microsoft Chief Executive Steve Ballmer. Microsoft watchers had speculated Mr. Sinofsky was a leading candidate to succeed Mr. Ballmer, but some people close to Microsoft said worries about Mr. Sinofsky’s inability to collaborate with other senior executives made him unlikely to become CEO.”

According to Ballmer Larson-Green – evidently in contrast to Sinofsky – is supposed to have “unique product and innovation perspective and proven ability to effectively collaborate and drive a cross-company agenda will serve us well as she takes on this new leadership role.”

In a statement Sinosky said, “It is impossible to count the blessings I have received over my years at Microsoft. I am humbled by the professionalism and generosity of everyone I have had the good fortune to work with at this awesome company.”

Ballmer said, “I am grateful for the many years of work that Steven has contributed to the company.” He probably wasn’t grateful for Windows neglecting to include the promised browser choice that may cost the company a packet in new fines payable to the European Commission.

Sinofsky worked with Office, then took control of Windows 7 after the dismal Vista launch and put out Windows 8 last month. Ballmer’s all-hands memo offered no whit of explanation.

Tellerm will report to Ballmer.