Adam Clarke on Genesis 43:18

And the men were afraid - A guilty conscience needs no accuser. Every thing alarms them; they now feel that God is exacting retribution, and they know not what the degrees shall be, nor where it shall stop.

Fall upon us - התגלל עלינו hithgolel alainu, roll himself upon us. A metaphor taken from wrestlers; when a man has overthrown his antagonist, he rolls himself upon him, in order to keep him down.

And our asses - Which they probably had in great number with them; and which, if captured, would have been a great loss to the family of Jacob, as such cattle must have constituted a principal part of its riches.