Similar to the settings of "crooked," "straight," et al in No. 3, here, there's a programmatic aspect with "shake." It's separated into many notes to give the impression of shaking.
At one point, "the heavens" has an rising melody, and "the earth" a falling one, as if to reflect the relative positions of each (the heavens are above the earth).
I didn't notice this until I started listening to Messiah while looking at the notation, but there's also a part where the instruments start playing after a rest, corresponding to "come" in "he shall come." It's like they're meant to emphasis the coming.
