I refuse to believe that he wasn't thrown under the bus in some capacity with all of the Xbox Live hacking issues. EDIT: Either that, or his leaving is tied to that in some capacity. This announcement just feels too convenient. But hey, if he wants to leave, good for him. I'm just pointing out a nagging feeling I have about this.
Again, this is all just speculation, but I'm more inclined he just left due to it, not that he was fired for it. From what he's said about the process and his limited resources, I can't imagine how he's dealt with the recent influx of trouble. He probably quit of of frustration with the system.