What does Judges 9:31 mean?
Zebul is the governor over Shechem. This implies that Abimelech's duties as lord of the surrounding region regularly took him away from the city. Or, possibly, that he simply preferred to let someone else handle day-to-day decisions. Zebul was probably not from Shechem originally, since Abimelech's rising rival, Gaal, pointed to him as evidence of Abimelech's outsider status (Judges 9:26–30).Either directly or through word of mouth, Zebul has heard Gaal's boasts. Possibly made brave by too much wine, Gaal has claimed he would remove Abimelech by force, if given control of Shechem. Zebul is furious, taking Gaal's words as a deadly threat. He wastes no time in sending a messenger to let Abimelech know about this challenge.
Zebul tells Abimelech that Gaal and his relatives are stirring up the city against him. It's not clear if Abimelech already knew this (Judges 9:25) or if this was his first warning. Whether he knew it or not, the leaders of the city were already standing against Abimelech's authority. Gaal isn't needed other than to put a public face to this rebellion. Zebul believes Abimelech must act before it's too late.