When Frank Darabont left the show, he worked hard to make it seem like AMC was the party at fault. That may have been at least partially correct, Frank wasn't exactly an angel. The network released thousands of emails from the showrunner that detail his angry and threatening language.
In the emails, Darabont expresses his frustration with his writers by saying that he should have "killed them with a brick and burned down their homes." He also used phrases like suggesting he should "start killing people and throwing bodies out the door." It seems like AMC may have had some cause to fire him.
Kirk McKoy / Los Angeles Times