From a joint statement from AMC and Glen Mazzara via Deadline:
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.
While I'm happy that a 4th season has been greenlit, what does this mean for the show, with Mazzara leaving, other than it'll get another showrunner?
Mazzara himself added in a statement that his time as showrunner was up, he'd told the stories he wanted to tell, and that it was time to move on.
Is there more to this than we're being told? More than likely, I'll say. But nobody's really talking yet, except for the above press statements; and in particular, the line about both sides essentially agreeing to disagree on the direction of the show.