John Gill Commentary Judges 9:30

"And when Zebul the ruler of the city heard the words of Gaal the son of Ebed, his anger was kindled." — Judges 9:30 (ASV)

And when Zebul the ruler of the city Whom Abimelech had placed there under him: heard the words of Gaal the son of Ebed, his anger was kindled; because he spoke slightly of him, and wished to have his place; perhaps before Zebul was inclined to be on the side of the Shechemites against Abimelech, or at least dissembled that he was; but now, being incensed at the words of Gaal, determined to take the side of Abimelech, and let him know how things were carrying on against him.