We are not told how soon after the death of Gideon these events happened. There must have been time for the apostacy and establishment of Baal-worship, and for the development of ill-will between Abimelech and his brethren.
Related Commentaries of Judges 9:1
Judges 9:1
1 And Abimelech the son of Jerubbaal went to Shechem unto his mother's brethren, and communed with them, and with all the family of the house of his mother's father, saying,