What makes you quiver? It’s a fundamental question.
Isaiah sees Yahweh enthroned, and hears the voice of thunder that makes the posts of the house to quiver (6:4). Elsewhere, Yahweh makes the idols of Egypt shake in fear (19:1), and the earth itself (24:20). The wine of His judgment makes nations quiver and stagger and fall (29:9).
That doesn’t move Ahaz. But when Ahaz hears about a confederation of Aram and Israel, his heard shakes within Him, and so too the hearts of all Judah (7:2).
Ahaz quivers at the anger of Rezin and Pekah (7:4), but their smoke and fire is nothing compared to the fire of Yahweh (6:1-4). Quiver at the anger of Yahweh, Isaiah tells the king (cf. 5:25).