First of all Samuel makes it clear that sacrifices, even sacrifices to YHWH, without obedience, are meaningless exercises. That is worth a whole sermon right there. Second, we get a closer look into just what idolatry is, into what the First Commandment prohibits. The text sets up a parallelism. It says the same thing in two ways.
Rebellion is like the sin of divination.
Opposition is like iniquity and idolatry. Saul's disobedience was rebellion. At the heart of it, of course, all disobedience is rebellion. This rebellion and opposition is likened unto divination and idolatry. Therefore, since disobedience is rebellion, and since rebellion is idolatry, then disobedience is idolatry.