The prisoners were sent off with men to guard them. In the meantime, Ammoron had finally sent new provisions and a large army to relieve Cumeni. They were too late for that task, but they found a somewhat depleted Nephite army and attacked. Into this heated battle returned the men who had been sent to guard the prisoners. There had to have been a fairly large number of them, as there were apparently many prisoners. If two thousand had been killed attempting to escape, there may have been at least that many remaining.
When the men who were sent to guard the prisoners returned, they found a battle raging, and fought, presumably from the rear of the Lamanite army, causing them to fight on two fronts simultaneously.