Albert Barnes Commentary Judges 9:1

"And Abimelech the son of Jerubbaal went to Shechem unto his mother`s brethren, and spake with them, and with all the family of the house of his mother`s father, saying," — Judges 9:1 (ASV)

We are not told how soon after the death of Gideon these events happened. There must have been time for the apostasy and establishment of Baal-worship, and for the development of ill-will between Abimelech and his brothers.