Why was Moses angry with his officers in Numbers 31?

Because they didn't kill all of the Midianite women.

Moses was wroth with the officers of the host, with the captains over thousands, and captains over hundreds, which came from the battle. And Moses said unto them, Have ye saved all the women alive? ... Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him. But all the women children, that have not known a man by lying with him, keep alive for yourselves. Numbers 31:14-18