“ And if we have not rather done this out of deep concern and with a purpose, saying, ‘In time to come your children might speak to our children, saying, “What have you to do with YHWH, the God of Israel? For YHWH has made Jordan a border between us and you, you children of Reuben, and you children of Gad. You have no portion in YHWH.” So will your children make our children cease from fearing YHWH.' ”
The Transjordan tribes clearly saw the altar as a symbol of their right to a presence in the land of promise. Their altar there, as it were, represented them. Thus in the future they would not be able to be told that they had no part in the land or in YHWH, for they now had their part within the border. Previously their rights had been preserved because their contingents were in the land fighting on behalf of YHWH, but now that they were leaving they felt that they must leave behind some presence in the land as a symbol of their right to a place in the covenant.
This brings out how deep an issue their settlement outside the boundaries of the land as promised had become to some in Israel. There are always those who cannot cope with change. It helps to explain why in Joshua we constantly find the emphasis on the fact that their settlement there was under command from YHWH, was their inheritance from Him, and was in accordance with the words of Moses (Joshua 1:14-15; Joshua 12:6; Joshua 13:8; Joshua 13:15; Joshua 13:24; Joshua 13:29; Joshua 13:32; Joshua 18:7). It was also seen as confirmed by the fact that they had Levites, whose inheritance was YHWH, living among them, which is specifically brought out by contrasting the inheritance in Transjordan with the inheritance of the Levites three times to bring out its genuineness (Joshua 13:14; Joshua 13:33; Joshua 14:3).
So the purpose of the altar, rather than being with the intention of breaking the tribal covenant, was in fact in order to ensure its continuation and to guarantee that they would not be excluded from it.
“Out of deep concern.” Consider the same word in Proverbs 12:25; Ezekiel 4:16; Ezekiel 12:18-19.
"In time to come" is literally ‘tomorrow' (see also Joshua 22:27-28; Joshua 4:6; Joshua 4:21; Exodus 13:14; (Deuteronomy 6:20). ‘A border between us and you.' Some Israelites would see this as emphasised by the importance YHWH Himself placed on the crossing of the Jordan (Joshua 3 and Joshua 4), forgetting that representatives of the Transjordan tribes had crossed over with them (Joshua 1:14; Joshua 4:12) and had placed the memorial stones (Joshua 4:4).