I was as shocked as the next guy to learn that Peter Moore was moving on to EA.
Obviously, EA made him an offer he couldn't refuse. Plus he's getting back into the business of making the blades instead of selling the razors. Prior to Microsoft, Moore supported Sega through its transition from console manufacturer to third party developer.
Clearly Moore's passion was a great benefit for Microsoft, but I believe that his strengths are toward leading software developers. Moore was first to admit that he didn't have a great understanding of the hardware and perhaps that gap may have been what inspired this move.
Check out this interview with Dan Hsu of Electronic Gaming Monthly, for a look back at Xbox 360's former champion.