The people that were subordinate to the Aztecs had multiple reasons to rebel against them and help the Europeans when they arrived, the two biggest reasons being:
1. Aztec rule with force and intimidation
The Aztecs were very brutal, and in order to put the subordinate tribes in control they very using force and intimidation on regular basis, and that was not seen fondly at all, so there was a lot of despise against them.
2. The Aztecs demanded too big of a tribute
Apart from being brutal, the Aztecs demanded very large tributes by the subordinate tribes. The tributes included food, labor force, soldiers, and people for sacrifice for their gods.
