“In that day.” “That day” is the “Day of Yahweh” (see commentary on Zeph. 1:7).
“and a great crashing from the hills.” The hills around Jerusalem had idols and temples for idols, and it is likely that the great crashing from the hills in the Day of Yahweh is the idols and idol temples being destroyed (cp. Ezek. 6:3-6, 13), as God foretold in Leviticus (Lev. 26:30-31).