Which multitudes will be like fine dust?
Rashi: Sancheriv's multitudes. Fire will consume them, and they will be like dust.
Rashi: Your multitudes will perish quickly, like fine dust goes when one blows it. Zarayich are the foreigners who camp against you, i.e. Machaneh Ashur.
Malbim: Dust is separation of a composition into fine parts. Sancheriv's multitudes will die around Yerushalayim, and dead bodies will remain.
Why does it mention "uch'Motz Over"?
Radak: The matter is repeated in different words.
Malbim: There are two attributes of dust over chaff. (a) Dust particles are parts of a composition; chaff is merely what is discarded. (b) Dust lies in its place, unlike chaff that the wind blows away. Melech Ashur and a few of his officers who remained and did not die, will be like chaff that passes. The wind will carry him to his land; he fell via the sword of his sons in his land.
What will be suddenly?
Rashi: This matter.
Radak: Their demise; in the morning they were found dead. The prefix Lamed in l'Feta is in place of a Beis. The same applies to "Haragti l'Fitz'i" (Bereishis 4:23), "Lifneichem l'Charev" (Vayikra 26:7) and similar verses. L'Feta is like Pit'om; the repetition is to strengthen the matter.
