Why did some natives fight for the french and some fight for the british?

In the United States during the revolutionary period, the Native Americans were not unified. They were very tribalistic. Therefore the reasons of which a tribe will be in alliance with the British or French can be for numerous reasons and is independent of each tribe. Some examples may be that fellow allied tribes are allied with the British, so they will also ally with the British, or maybe the trading deals with the French is more beneficial to their tribe. Or maybe a tribe has better connections and politics with one country over the other. The point being that they where each their own entity and where not under a unified cause. So their allies were not unified as well. 

The answer should be europe