John Gill Bible Commentary

I beheld the mountains, and, lo, they trembled,.... At the presence of God, at the tokens of his displeasure, and at his awful vengeance in the destruction of the Jews, as they are sometimes said to do, Psa 68:8, and all the hills moved lightly; so Kimchi's father says the word used has the signification of lightness; though Jarchi, from Menachem, explains it, they were plucked up, and thrown out of their place; and some render it, were pulled down and destroyed, so the Targum. Mountains and hills are most stable, and not easily moved, wherefore this is said, to aggravate the desolation and destruction.