Via Joint Statement from AMC and Glenn Mazzara:
December 21, 2012 –
Today, AMC announces the season 4 pick-up of The Walking Dead.
AMC also jointly announces with Glen Mazzara today that for future seasons, the two parties have mutually decided to part ways. Glen guided the series creatively for seasons 2 and 3. AMC is grateful for his hard work. We are both proud of our shared success.
Both parties acknowledge that there is a difference of opinion about where the show should go moving forward, and conclude that it is best to part ways. This decision is amicable and Glen will remain on for post-production on season 3B as showrunner and executive producer.