1)
What is "Motz"?
1.
Radak: It is the fine straw that the wind blows.
2)
Why does it say "u'Mal'ach Hashem Docheh"?
1.
Radak: Sometimes Motz stands up against the wind, when it finds a fence or something to support it. They will not stand, for Mal'ach Hashem repels them.
2.
Malbim: They will be punished via an external cause, like chaff driven via wind, and not naturally. Mal'ach Hashem will repel them.
