The specific location of this altar—'on the frontier of the land of Canaan, in the region about the Jordan'—was a major point of concern for the other tribes.
The geographical placement of the altar is key to understanding the Israelites' alarm. It was built 'over against the land of Canaan, in the borders of Jordan, at the passage of the children of Israel.' This meant it was situated on the eastern side of the Jordan River, in the territory already granted to the tribes of Reuben, Gad, and half of Manasseh, but crucially, at a point that was the very gateway into the promised land for those coming from the wilderness.
For the tribes living west of the Jordan, this location represented a boundary issue. It was too close to their territory and too close to the established worship center at Shiloh. The fear was that this altar, so visibly positioned at a major crossing point, could become a focal point for a separate religious practice, effectively dividing the nation and undermining the singular worship mandated by God.
It underscored the anxiety that any deviation, especially one that seemed to create a religious 'other,' would fracture the unity God desired for His people. The altar's position made it a symbol that could easily be misinterpreted as a sign of separation, rather than the intended sign of connection.