He needed to start a new company to finish working on his Kinect-powered pedo simulator.
Seriously though, I believe he felt somewhat drained and needed a new, fresh start. It was the same with Bullfrog - they released alot of good games and then came a period of stagnation. Lionhead hasn't released a "truly amazing game" since Black and White and Fable - everything else was just re-hashed ideas. I fear that the creativity of Peter may have simply ran out... alternatively, he needs a "push forward".
When you have IP's that sell great such as Fable, you don't feel the "need" to think creatively anymore - you just release sequels. Maybe Peter wanted to leave all those trademarks behind him and start something completely new.
He's been in the industry for years and he took part in making games that were fantastic and games that were apalling, so... we can expect just about anything from him. He's one of those authors who are visionaries - they come up with glorious ideas that meet either universal approval or... become universally hated.