'Rashi': This is an expression of rebellion, like "va'Yifsha Mo'av" (Melachim II, 1:1), "Ach Nifsha mi'Kiryas Oz" (Mishlei 18:19), "Az Tifsha Livnah" (below, 21:10). Malbim - they rebelled absolutely. Now they were obligated to set up a king to oppose Rechav'am, who wants to force them.