Albert Barnes Commentary 2 Chronicles 31:1

"Now when all this was finished, all Israel that were present went out to the cities of Judah, and brake in pieces the pillars, and hewed down the Asherim, and brake down the high places and the altars out of all Judah and Benjamin, in Ephraim also and Manasseh, until they had destroyed them all. Then all the children of Israel returned, every man to his possession, into their own cities." — 2 Chronicles 31:1 (ASV)

Jerusalem had been cleansed (2 Chronicles 30:14), and now the land had to be purged. Hezekiah, therefore, gave his sanction to a popular movement directed as much against the “high places,” which had been maintained since the times of the patriarchs, as against the remnants of Baal worship or the innovations of Ahaz (see the note on 2 Kings 18:4).

The invasion of the northern kingdom, “Ephraim and Manasseh,” by a tumultuous crowd from the southern one, and the success the movement achieved, can only be explained by the weakened state of the northern kingdom (see the note at 2 Chronicles 29:24).