Answer:
The correct answer is : Because he tells everyone that he is the cause for the plague.
Step-by-step explanation:
Because Calchas calls him out and tells everyone that Agamemnon is the cause for the plague. Also, he felt insulted by Calchas because the prophet asked him to give up his war prize. He didn't admit his mistake and the king became monstrous and demanded compensation for what should not have been his in the first place.