On account of the Jordan River being a natural barrier as well as the (apparent) underlying assumption amongst the western tribes that the land to the east was not truly part of Canaan, the eastern tribes certainly had reason to fear that they might be excluded in the future. The fact that they built an altar is an implicit condemnation of western attitudes and an attempt to illustrate that both east and west are committed to worshipping the Lord only.1
24 No, but we did it from fear that in time to come your children might say to our children, ‘What have you to do with the LORD, the God of Israel?