NFL head coaches have one of the hardest jobs in the world in terms of pleasing your bosses. Unlike most people, head coaches don't just have to deal with the person who signs their check, they also have to deal with fans all over who are upset with what the coaches do.
With that, fans can get very vocal and can make it clear that the coach no longer has public support.
Sometimes this seems fair, in a case where a head coach has been with the franchise for years and has been in near complete control of what has been happening. Sometimes it isn't, like when the coaches' teams suffer from injuries to key players, loss of coaching staff or circumstances beyond their control.
These are a few coaches that, for whatever reason, may get fired as the season goes on.