Matthew Poole and continuators on Judges 11:5

5 And it was so, that when the children of Ammon made war against Israel, the elders of Gilead went to fetch Jephthah out of the land of Tob.

By direction or instinct from God, who both qualified him for, and called him to the office of a Judge. See Iudg. 12. 7. Heb. 11. 32. Otherwise they might not have chosen a Bastard, Deut. 32. 2. Unless we will say, that there being no other person among them fit for, and willing to this Work, necessity dispensed with this Law, as it did with other positive Laws, as those of the Sabbath and Sacrifices.