“ However, if the land of your possession seems to you unclean, then pass you over to the land of the possession of YHWH, in which YHWH's Dwellingplace (Tabernacle) abides, and take possession among us. But do not rebel against YHWH, nor rebel against us, in building yourselves an altar besides the altar of YHWH our God.”
It was probably the fact that the altar had been built on the side of the Jordan belonging to the land of Canaan that gave them the impression that the Transjordan tribes had done this because they thought that their own land was unclean, that is, not totally separated to YHWH, and not hallowed by YHWH's presence. It was not ‘His land'. If that was their view (and it was probably the view of many of the Israelites who lived in Canaan) then they had only to come across the Jordan and allotment would be given to them so that they could live in the land that belonged to YHWH, where YHWH had His dwellingplace. But let them not rebel by building a false altar.
It was clear that they saw this altar as not one that YHWH had caused to be built by revelation, it was not in a place where YHWH had recorded His name. Thus it was a sacrilegious altar. The main altar of YHWH was that which accompanied the Tabernacle at the central sanctuary. Others could be built where YHWH revealed Himself and commanded it. These came within the definition of ‘the altar of YHWH our God'. But not this one where there was no suggestion of YHWH having spoken.
Note the stress on rebellion. They were rebelling against God because they were disobeying His command about building altars where He had not given a revelation, and they were rebelling against their brothers because they were setting up a rival altar to that of the central sanctuary and thus breaching the covenant unity.