The people that were subordinate to the Aztecs had multiple reasons to rebel against them, and the biggest ones were that the Aztecs were very violent, and they were demanding to much tribute to be paid by the people they were ruling over.
The Aztecs were so feared by the people subordinate to them, that even without any military action, just mentioning or threatening that there's going to be one, got every scared and silenced. When the Spanish came to Mexico, the people subordinate to the Aztecs saw an opportunity to get away from their rule, so they didn't hesitate at all to join and help the Spanish, and it resulted in the fall of the Aztec Empire.
