Thomas Gibson starred on Criminal Minds as "Hotch", the team leader of the "BAU". He successfully closed one case after another, could always rely on his team and therefore always protected them as best he could. 

But all of this suddenly ended. The reason: At the beginning of season 12 in 2016, there was an altercation on set between Gibson and the producer and screenwriter Virgil Williams. According to CNN there was an actual physical fight between the actor and his producer.

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After the incident, Gibson was immediately fired from the show, since it wasn't the first time he had to be reprimanded and he was already in anger management classes.

Criminal Minds: "Hotch's" exit from the series

Of course, it wasn't easy to quickly rewrite the season 12 scripts after having "Hotch" on the show for 11 seasons, but the producers found a solution. Right at the beginning of season 12, "Hotch" was sent on a special assignment.

Also, Paget Brewster was cast as "Emily Prentiss", who then appeared at the beginning of the third episode. In season 12 episode 6 ("Elliott’s Pond") it was explained what happened to "Hotch" in Criminal Minds and why he would not return.  

Criminal Minds: "Hotch" had to protect his son

His special assignment was only a cover story from the start, as "Rossi" (Joe Mantegna) explained to his colleagues in the episode. The serial offender "Peter Lewis" (Bodhi Elfman), also known as "Mr. Scratch", actually broke out of prison and was soon after spotted at one of "Hotch's" son "Jack's" (Cade Owens) soccer games.

The FBI then put a surveillance team on "Jack", but after "Mr. Scratch" also showed up at the boy's school, "Hotch" and his son decided to go into a witness protection program.

This meant that "Hotch" was no longer allowed to have contact with anyone from his previous life, including his colleagues. So, Thomas Gibson's character left the FBI to protect his son "Jack". His last wish: "Prentiss" should be his successor and lead the team. And she accepted the job.