And king Arad … heard of the coming. See on Numbers 21:1. The introduction of this notice, for which there seems no motive, and which has no assignable connection with the context, is extremely perplexing.
It is not simply a fragment which has slipped in by what we call accident (like Deuteronomy 10:6, Deuteronomy 10:7), for the longer statement in Numbers 21:1-3 occupies the same position in the historical narrative immediately after the death of Aaron.
It is difficult to suppose that Moses wrote this verse and left it as it stands; it would rather seem as if a later hand had begun to copy out a statement from some earlier document—in which it had itself perhaps become misplaced—and had not gone on with it.