I read an article, I can't remember where, that said he didn't fire them. According to this article Darabont was only supposed to do one season and then his right hand man, Charles Eglee, was going to take over. Well Darabont went to AMC and said he'd like to stay on and of course AMC was thrilled so Eglee decided to leave because he wanted his own show. And some of the writers went with him.
Either way I'm glad Darabont is going to stay on.