I was trying to track down an error similar to this last year. I never fully confirmed it, but Streamline's "corpse killer" function seemed to be the culprit. Every time it issued a "kill" command to an already dead creature/NPC it seemed to give them more items.
Interesting. I'l ltry turning off that function and see if it makes a difference. It doesn't seem to work all that well anyway, I still see standing corpses from time to time.
PS:. It doesn't look like that's the problem. I turned it off and went back to Plundered Mine, yet the same thing still happened. In fact, I once again got the exact same death inventory as the one I listed. If it was Streamline, I suspect it would be a bit more random, but it's always the same. And why would it only affect the Berserkers? The regular Goblins are just fine.
I noticed something similar happening when I did the Mehrune's Razor quest. In a couple of areas, the Village and the entrance to it, any Bowman I killed would end up with a bunch of arrows, far more than a character normally has. In the other areas though, like the forge and mines, they were normal. I ended up getting out of there with almost 1500 Iron arrows, plus a bunch of other types. I probably had about 3000 arrows in total by the time I was finished that quest. Plus an insane amount of other loot. It took me at least 3 game days just to haul it back and get rid of it all. I think they made that area a little too big frankly. Interesting though.