ONE reason why the people subordinate to the Aztecs were ready to rebel when the Europeans arrived.

Yes, what the person above said

Over taxation on part of the Aztecs, for years the aztecs exacted cumbersome tributary requirements on their subordinates, such as demanding xx amount, (could be in the thousands) of young men and women in order to sacrifice them. This served to also bleed the affected tribes of people, keeping them weak and perhaps even dependent on the Aztecs for protection and continuity.