The reason is clear: this is a single army proceeding in one direction, so they become partners.
His statement, “The one who unlawfully takes from the booty”: this is the person who conceals something of what he has taken as booty and keeps it for himself. Such misappropriation is among the major sins. Allah says:
“It is not for any prophet to act dishonestly regarding the spoils; whoever acts dishonestly will bring forth what he misappropriated on the Day of Resurrection.” [Āl ʿImrān 3:161]
The Prophet warned against this and explained that a person will come on the Day of Resurrection carrying whatever he unlawfully took—whether a sheep, a camel, or anything else—before all people. (1) It is therefore among the major sins. The Messenger also mentioned that a man had unlawfully taken a cloak and said: “...Indeed, the cloak which he took from the spoils on the day of Khaybar, before it had been apportioned, will be set ablaze as a fire upon him.” (2) We seek refuge with Allah.