Writer-director Frank Darabont (The Shawshank Redemption) has stepped down as showrunner of AMC's The Walking Dead, halfway through production on the new 13-episode second season. His replacement is expected to be Glen Mazzara (showrunner of Starz's Crash), who's been working as Darabont's second-in-command after joining the show with a group of new writers—after Darabont disbanded the first season's team. (Update: this has now been confirmed as happening.)
It's not been revealed why Darabont has felt the need to quit, but he's been unhappy with AMC's plans to trim the show's budget, and there were rumours he found it hard to adjust to the demanding pace of TV production (as he comes from a movie background), but this is all just speculation. I wouldn't be surprised if there's some truth in all that, however. The show clearly struggled to produce six episodes last year, so doubling the order to a standard thirteen may have felt particularly gruelling for Darabont.
I have a feeling Darabont would have been happier making The Walking Dead as a series of movies, doesn't like the time/money constraints of television, and perhaps is too authorial to enjoy working in a writers' room. But who knows. Hopefully Darabont will release a statement soon—but those things are usually misleading and don't get at the truth behind these matters. We probably won't know what happened for a few years or more.
It's speculated that Darabont could retain some kind of credit on the show, but without him to steer the ship it'll be interesting to see how The Walking Dead changes halfway through season 2. Hey, it may even improve...