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For reasons that no one understands so far, and may never be clear, the head of Microsoft Corp.’s (NASDAQ: MSFT) Windows division, the company’s flagship, left suddenly without a farewell party or a proper send-off. Windows and Windows Live President Steven Sinofsky disappeared in the blink of an eye. Someone named Julie Larson-Green replaced him.

There are several theories about why Sinofsky left. Two are the most likely to be true. Windows 8 sales may be well below Microsoft’s expectations. The company’s future hings on the success of the radical relaunch of Windows, which Microsoft claims will be a success across platforms as broad as PCs, smartphones, game consoles and tablets. Sales of Microsoft’s new tablet — the Surface — already have been described by CEO Steve Ballmer as “modest.” Detailed sales figures for Windows 8 will not be given to the public right away. Sinofsky may have taken the blame before any public disclosure of these.

Another theory, one which is more conspiratorial, is that Sinofsky was the most likely candidate to replace Ballmer. If Windows stumbled, he was in the wings, with years of experience at the world’s largest software maker, to take over. Since the board has supported Ballmer through very trying times, this would seem to be the less plausible of the two.

Ballmer’s comment:

“I am grateful for the many years of work that Steven has contributed to the company,” Ballmer said. “The products and services we have delivered to the market in the past few months mark the launch of a new era at Microsoft. We’ve built an incredible foundation with new releases of Microsoft Office, Windows 8, Windows Phone 8, Microsoft Surface, Windows Server 2012 and ‘Halo 4,’ and great integration of services such as Bing, Skype and Xbox across all our products. To continue this success it is imperative that we continue to drive alignment across all Microsoft teams, and have more integrated and rapid development cycles for our offerings.”

Douglas A. McIntyre