Albert Barnes Commentary Judges 3:12

"And the children of Israel again did that which was evil in the sight of Jehovah: and Jehovah strengthened Eglon the king of Moab against Israel, because they had done that which was evil in the sight of Jehovah." — Judges 3:12 (ASV)

The “strengthening” Eglon was the special work of God, and because Israel had done evil, etc. Samuel’s comment on the event is to the same effect (1 Samuel 12:9).