After Gaal’s expulsion, “the people went out into the field,” either to complete the vintage, or for some other agricultural operation. “They” (Zebul and his party) sent word of this to Abimelech.
Related Commentaries of Judges 9:42
Judges 9:42
42 And it came to pass on the morrow, that the people went out into the field; and they told Abimelech.