“ Is the iniquity of Peor too small for us, from which we have not cleansed ourselves to this day, although there came a plague on the congregation of YHWH, that you must turn away this day from following YHWH? And it will be, seeing you rebel today against YHWH, that tomorrow he will be filled with wrath against the whole congregation of Israel.”
Israel ever remembered the sin of worshipping Baal-peor (the lord of Peor), which was probably another name for Chemosh, the national god of Moab, as a result of the seducement of the daughters of Moab (Numbers 25:1-3). It was a stain never completely removed, even though a plague from YHWH had followed (Numbers 25:8-9) which was only stayed by the action of the same Phinehas as is mentioned here (Numbers 25:6-8; Numbers 25:11). Now they were afraid that the action of the Transjordan tribes would bring a similar plague on them all.