Why was Chiron fired in the sea of monster book?

Chiron was not fired in *The Sea of Monsters*. He is still the headmaster of Camp Half-Blood in that book. He is, however, forced to resign as headmaster in *The Titan's Curse* due to a prophecy that states that the next time a god walks onto the grounds of Camp Half-Blood, they will bring destruction.

Since Chiron is a centaur, he is half-god, and the prophecy applies to him. To protect the camp and his students, he steps down from his position.