And the people of Israel heard it said, "Behold, the people of Reuben and the people of Gad and the half-tribe of Manasseh have built the altar at the frontier of the land of Canaan, in the region about the Jordan, on the side that belongs to the people of Israel."
Joshua 22:11

Building an altar on a piece of land could be interpreted as a declaration of ownership. The western tribes' first concern seems to have been that the eastern tribes were attempting to colonize them. They already considered the eastern tribes to be a separate people.

The author divided the people into "Reuben, Gad, and the half-tribe of Manasseh" on one side and the "people of Israel" on the other. This wasn't because they were actually separate peoples, but to to emphasize the division that was taking shape in the minds of the ten tribes on the west. Although the complaint began as a territorial dispute, purity of worship became the paramount issue.

The altar had a dual purpose: To keep the west from treating the east as foreigners and to keep the east focused on YHVH.