The bullock was offered Not upon Baal's altar, for which it was designed, but upon an altar erected in contempt of Baal. When they inquired, they said Probably some of the persons employed in it.
Related Commentaries of Judges 6:28
Judges 6:28-29
28 And when the men of the city arose early in the morning, behold, the altar of Baal was cast down, and the grove was cut down that was by it, and the second bullock was offered upon the altar that was built.
29 And they said one to another, Who hath done this thing? And when they enquired and asked, they said, Gideon the son of Joash hath done this thing.