The translation “stirring up” is undoubtedly correct, even though the verb in question has this meaning nowhere else and always means “besieging” when used of a city. The given translation takes its cue from this, however. For Gaal and his brothers were indeed making an assault against the city and forcing it to turn against Abimelech, not by a siege from without, but by the power of persuasion.
31 And he sent messengers to Abimelech secretly, saying, “Behold, Gaal the son of Ebed and his relatives have come to Shechem, and they are stirring up the city against you.