Why the change? According to Gaffney's post, part of it is a result of both losing several family members to cancer last year and having to receive treatment for skin cancer himself. He's recovering without incident, but he put off the treatment for nine months to keep working through launch. Meanwhile, he also wants to allow the studio's creative team to keep working on improving the game rather than simply listening to his creative vision, something which is more easily accomplished if he steps away from an active role.
Gaffney concludes his letter by thanking the fans as well as the studio for making a game that he's proud to have been a part of.
[Thanks to Pete for the tip!]