It most likely means that Sihon did not trust Israel’s motives to pass through in peace.
Israel won the battle and so won the land by right of conquest (Judges 11:21). The land was never the Ammonites’ (who lived north of the Jabbok), and it was won by Israel fairly from the Amorites.
20 but Sihon did not trust Israel to pass through his territory, so Sihon gathered all his people together and encamped at Jahaz and fought with Israel.