Albert Barnes Commentary Numbers 31:16

"Behold, these caused the children of Israel, through the counsel of Balaam, to commit trespass against Jehovah in the matter of Peor, and so the plague was among the congregation of Jehovah." — Numbers 31:16 (ASV)

Caused ... to commit trespass - More literally, “became to the children of Israel for a cause (or, incitement) of treachery to the Lord.”