Because he turned down the extension, Brown spent all year in the media critiquing him, Fox's new agent then had a meeting with managers about the future, then Fox had some bad performances capped with a game losing boneheaded play that was the opposite of what he was told, Brown then singled him out even more, and within a day he was fired. And Fox sure wasn't seeming to have any problem with the firing when talking to the media after. Said his reaction to being told on the team plane that Brown was fired was to just go to sleep.
Might not be fair to think he's a coach killer but there's enough to raise eyebrows.