I doubt that is extra souls, it's most likely all the souls from the urn exiting. The person who placed the Urn would have had them all if they disappeared on their own, since you can steal souls it's most likely you can steal all the souls from the urn if the person doesn't stick around to shot or melee them before the enemy. This opens up some major dynamics around it, safe to place with no one around otherwise be prepared to share.
From what i can tell on my clip part of the souls get distributed evenly to the team inmediately once you return the urn and there's quite a few that are given extra to the person that returns the urn, i got like 800 from that one alone lmao