Why did Poseidon side with the Greeks?

He was originally on the Greeks side. He and the other gods tried to over throw Zeus. He was caught, but escaped. For Poseidon's and Ares punishment, they were forced to 1 year of labor to the Trojans. The king promised to pay them much money, but refused after their labor. Poseidon then sided with the Greeks and sent a vicious sea monster on the Trojans.