Although, it's called the French and Indian War, it was not a one-sided war with the French and Indians versus the British. In reality, it was the French and their Indian allies versus the British and their Indian allies. Colonists also fought on both sides, but for this sake let's just equate them with their respective mother countries.

My research mainly lies within Pennsylvania, but I can give enough info to cover the rest. Now, to understand these questions we need to look back in the history just a bit. The main colonial powers in North America were of course the British, the French, and the Spanish; however, we'll be looking at the former two for this. Indians played a huge part in the politics of the British and French. France heavily relied on their native allies, as the French did not have the numerous population the British colonies had. The Ohio Valley region was a hotbed of contention between the French and British, as it was a lucrative point for the fur trade. Native tribes of the Ohio region like the Ojibwe and the Iroquois Nation stood in between France and Britain, often playing the two powers against each other.

Older historians from the earlier Twentieth century and Nineteenth century treated Indians as mere pawns and obstacles to the great European leaders. This has changed with more revisionist histories, giving Indians a voice. Though the contested lands were bickered over by the French and British, let us not forget that Indians inhabited those very lands.

Over the course of decades, British colonists crept westward and pushed tribes like the Delaware and Shawnee out. In 1754, with the treaty of Albany, the Iroquois sold the lands out from under the Delaware's feet, causing them to move westward again. This wasn't the first time, as in the 1730s, the Pennsylvania government dubiously bought Delaware through the Walking Purchase, where the government was to buy all the land in the amount that could be walked--so they bought trained runners. The Delaware had also been facing tensions with the Iroquois Confederacy. The Delaware did not have the same sovereignty as the Six Nations did in the Confederacy and were technically dependents, which is why they had little say in the land dealings between the Iroquois and the British. The grievances of land and sovereignty made it more than enough for the Delaware to pick up the hatchet against Pennsylvania colonists, as they not only took vengeance for the land grabbing, they also acted in defiance of the Six Nations.

Tensions between Algonquin and Iroquois also existed. The Iroquois had been encroaching on Algonquin land in the Ohio Valley and the Great Lakes region. The Algonquins (officially under the Seven Nations of Canada) sided with the French and fought against the Iroquois. So not only did Indian tribes side with the French to ward off British colonists, they also did so against other Indian tribes.

The Iroquois was a dominant power in the Northeast (concentrating what is now NY, PA, parts of Ohio and Canada). However, they remained mainly neutral throughout the war and acted mainly as a mediator between the Delaware and Pennsylvania. The British did manage to find alliance with the Cherokee and Creek, who had also fought against the Algonquins in previous histories. The Cherokee and Creek played a monumental role in Forbes' 1758 Expedition to Ft. Duquesne (later became Ft. Pitt after the British took over, now Pittsburgh). Colonial rangers often fought side by side with their Cherokee allies against the French and their own Indian allies. At least, until the British had a falling out with the Cherokee in 1759 and started the Anglo-Cherokee War.

I hope you're still with me. The war was actually more complex than it seems, and Indian politics can be a little tricky to comprehend. Each nation had their own reasons to side with a European power. For some like the Delaware, it was British encroachment and sovereignty; for some like the Cherokee, it was alliances and a chance to kick some Algonquin ass--to put it simply and colloquially.

And then it gets even more interesting by 1763 where the Ohio region Indians can't play the French and British off of each other anymore. But that's another matter.

Please, if there's anything I need to add or clarify, just ask. I've got shitloads of info on this in my head and it's hard to get it all out on one little post.

For further reading, I'd really recommend Francis Jenning's "Empire of Fortune" and Richard White's "The Middle Ground." They'll give you both the proper background and show you why many Indian tribes acted the way they did.