Why leave a game that's still early in testing and doing well? As Hall puts it, keeping him on the project could eventually lead to his being someone who tells others how things are done rather than adapting along with a changing game and culture. Instead, he's heading to New Zealand with plans of opening a different studio. The changeover is not happening immediately but should take place before the end of the year.
[Thanks to Zipzopboobidybop for the tip!]