According to a report from All Things D,
There were several reasons for the departure, which was amicable and has been planned for a while, sources said. They included Kittlaus’ family being in Chicago, a desire to take time off and an interest in brainstorming new entrepreneurial ideas.
Kittlaus was the lead for Apple during Siri’s introduction to the4S and has been Siri’s CEO since April 2007. He apparently left Apple just after the launch of the .
Source: All Things D