The people on the bridge is there regardless of whether the three leave the grove or stay. They're either supposed to be unrelated people or that's where the original ambush took place.
That's more or less what my take's been. They got routed at the goblin camp, and then followed as they fled. The bodies by the bridge are the comrades who they lost in a skirmish as part of their retreat to the grove. When we encounter Aradin there, it's his party's first opportunity to visit their dead.
