Can someone explain Aztec cannibalism from the standpoint of ecological stress?

I thought Aztecs only participated in cannabalism during rituals. I don't think they were eating other humans because they ran out of food. Maybe they had more rituals when times were tough (drought, flood, famine, etc.) to seek help from their gods and during those rituals engaged in cannibalism. From what I understand, the human sacrifice was the largest "gift" a person or a group could give to a god.