>so I check out the frames where
>they came from and find larva and capped WORKER brood with a smattering of
>drone brood.

If it is CAPPED brood then it could not have come from one weekend to the
next and MUST have been there on your first inspection and you missed it.
The bees are defensive because of the robbing.  Equalize the strength
between the two hives by transferring some frames of brood ready to hatch.
You could also shake in some young nurse bees if the hive is too weak to
keep the brood warm.  Keep the entrance shut to the minimum to help them
defend themselves.

Regards
Stan