Why might the eastern tribes have been worried that the western tribes’ descendants would seek to exclude them?
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 C'est bien plutôt par une sorte d'inquiétude que nous avons fait cela, en pensant que vos fils diraient un jour à nos fils: Qu'y a-t-il de commun entre vous et l'Eternel, le Dieu d'Israël?