Simply put, they broke God's commands and served other gods. This continued right on through Samuel's period of judging. During Samuel's period of judging, God permitted them to have a king (Saul), who also proved to be unfaithful. Saul was what we would call today in the political realm a pragmatist. He simply did what seemed right. That is, he did what seemed to work at the time. But there was no real character there. There was no hewing to the straight and narrow of God.
