‘ And they went out into the field, and gathered their vineyards, and trod the grapes, and held a festival, and went into the house of their god, and ate and drank and cursed Abimelech.'
Gaal stayed until the time for gathering the grape harvest came, and when the harvest was gathered they all held a festival, as would occur yearly at that time, and began to enjoy the fruits of the vine. In process of it they went into the house of their god, Baal-berith, eating heartily and getting drunk, and in the course of this, their tongues running freely, they cursed Abimelech. They had become disillusioned with him. This probably includes the idea that they discussed ways of getting rid of him. He was getting in the way of their profitable highway robbery. Gaal would have listened to this with interest.
Of course at this time they should have been gathering at the central sanctuary to worship Yahweh at the feast of Tabernacles. That may well have been where Abimelech, as a prince of Israel, was. But their support of that covenant was now non-existent.