The leaders of Shechem set up an ambush against Abimelech on the hilltops, and they robbed all who passed by them on the road. So this was reported to Abimelech.

Judges 9:24
in order that the crime against the seventy sons of Jerubbaal might come to justice and their blood be avenged on their brother Abimelech and on the leaders of Shechem, who had helped him murder his brothers.

Judges 9:26
Meanwhile, Gaal son of Ebed came with his brothers and crossed into Shechem, and the leaders of Shechem put their confidence in him.

And the men of Shechem set liers in wait for him in the top of the mountains, and they robbed all that came along that way by them: and it was told Abimelech.