What "Ma'asecha" did they trust in?
Radak: Their flocks and property. This is like "v'Ish b'Ma'on u'Ma'asehu va'Karmel" (Shmuel I, 25:2).
Radak citing Targum Yonasan: Your storehouses and treasure houses.
Malbim: Future success in the field and vineyard. "Otzerosayich" is what you already gathered.
Why does it say "Gam At"?
Radak: Also you will be trapped, like the other nations who do not have storehouses like you, and they did not have serenity for a long time like you had - "Sha'anan Mo'av" (verse 11). Your storehouses that you trusted in will not help you, nor your god Kemosh. Also it will be exiled!
Malbim: (a) You will not want to flee and abandon your Ma'asim in the field and your storehouses in the city in which you put all your trust. Therefore, not only will [the enemy] take your storehouse - also you will be trapped. (b) Also Kemosh, its officers and priests will be exiled. Therefore, it is good that you flee and abandon your property, and at least you will save your lives!
