While Judah enjoyed stability under the longevity of their good kings, Israel was swapping monarchs hand over fist. In Judah, Azariah (that is, Uzziah), Jotham and Ahaz ruled for a combined period of over 8 decades. Meanwhile Israel endured Jereboam II, Zachariah, Shallum, Menahem, Pekahiah, Pekah and Hoshea, along with the violent upheavals which accompanied several of the transitions of power there. The contrast is not coincidental. Israel was base and astray morally and spiritually. Judah wasn't perfect, but they were still comparatively righteous (as a nation).
The proof that the northern kingdom was crossing a Rubiconic line is the introduction of the Assyrians onto the scene here. Syria had been troubling Israel for decades, but now Assyria (the empire) was a threat. It started with money taken by Pul (II Kings 15:19-20) and progressed (or digressed) to land and people taken by Tiglathpileser (II Kings 15:29).