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.
Related Commentaries of Judges 3:12
Judges 3:12
12 And the children of Israel did evil again in the sight of the LORD: and the LORD strengthened Eglon the king of Moab against Israel, because they had done evil in the sight of the LORD.