Or HaChaim on Numbers 21:12/אור החיים על במדבר כ״א:י״ב

1 משם נסעו, From there they journeyed, etc. We need to understand why the Torah changed its style in describing the journeys of the Israelites by introducing both this journey and the one following it as משם, "from there." The Torah's normal way of telling us about the Israelites' journeying has always been the word ויסעו. Perhaps the reason is that the two journeys introduced by the word משם נסעו, i.e. the journey from the river Zered and from the river Arnon occurred at the Israelites' own initiative seeing there were no longer any clouds signalling their move. I have seen proof that my estimate is correct by what is written in Deut. 2,13: "now rise up and get over the brook of Zered, etc." These words clearly indicate that the Israelites did not journey at the behest of the clouds of glory. If the journey had started because the clouds moved, why would Moses have to tell the people to get moving? Although the Torah had said in Numbers 9,20 that the Israelites made camp and broke camp at G'd's instructions, this referred to all the other journeys barring these two. The reason was that Aaron had died and the clouds of glory had disappeared. Even though we have been told in Taanit 9 that these clouds re-appeared due to the merit of Moses, it is possible that the particular cloud which signalled to the Israelites that they were to break camp had not returned.