Charles Ellicott Commentary 1 Kings 12:20

"And it came to pass, when all Israel heard that Jeroboam was returned, that they sent and called him unto the congregation, and made him king over all Israel: there was none that followed the house of David, but the tribe of Judah only." — 1 Kings 12:20 (ASV)

Jeroboam was come again. —The assembly at Shechem probably broke up in disorder, carrying the news of the rebellion everywhere. It would be quite consistent with Jeroboam’s astuteness if, after setting the revolution in motion, he himself remained aloof from leadership and waited until “the congregation,” the duly summoned assembly, sent for him and offered him the crown.

The title “king over all Israel” certainly indicates a claim by the ten tribes to be the true Israel, relying perhaps on the prophetic choice and blessing of Jeroboam, and professing to have risen in the name of the Lord against the idolatry of Solomon and his house. Perhaps it also indicated a desire for the subjugation of Judah, which Jeroboam, with the aid of Shishak, certainly seems to have later attempted.