Sinofsky's departure comes just weeks after Microsoft launched Windows 8, a major overhaul of the operating system that's used on most of the world's computers. The Windows 8 was met with lackluster reviews.
Citi Investment Research analyst Walter Pritchard said he was surprised by the departure, likening it to a football team changing quarterbacks in the middle of the playoffs.
He said he'd assumed that Sinofsky, whose duties will be taken over by two executives, would stay at Microsoft to see through the project—Windows 8—that he had led.
"The move itself is a surprise since many saw Sinofsky as a possible successor to (Microsoft CEO) Steve Ballmer, although with Windows 8 released the timing isn't," Pritchard said. Sinofsky, he added, had a reputation for a "regimented, inflexible and focused approach to development that pre-dated his Windows tenure."
Shares of Microsoft Corp., based in Redmond, Wash., fell $1.24 to $28.22 in premarket trading.