Charles Ellicott Commentary Judges 9:31

"And he sent messengers unto Abimelech craftily, saying, Behold, Gaal the son of Ebed and his brethren are come to Shechem; and, behold, they constrain the city [to take part] against thee." — Judges 9:31 (ASV)

Secretly. —The Hebrew is betormah, which may mean “to Tormah,” or Arumah, where Abimelech was living (Judges 9:41). The word occurs nowhere else, and the versions differ (Septuagint, in secret; Codex B, with gifts; Codex A reading batherumah). Whether “craftily” is the right rendering or not, it is clear that the message was a secret one, for Zebul dissembled his anger until he was strong enough to throw off the mask.

They fortify. —Rather, perhaps, they tyrannize over the city because of you.