I asked Dave Nowitz, one of the authors of HoneyDanBer UUCP (the others being Peter Honeyman and Brian Redman) if he knew what bag files were. He said

Never heard of bag files.
Maybe whoever wrote the linux version did it?

On Wed, May 9, 2018 at 11:51 PM, Lyndon Nerenberg <lyndon@orthanc.ca> wrote:

> On May 9, 2018, at 8:49 PM, Grant Taylor via TUHS <tuhs@minnie.tuhs.org> wrote:
>
> Does anyone know why UUCP "bag" files are called "bag"?

What are they to begin with?  Having run some "major" UUCP hubs in the day, I have no clue what a UUCP "bag" is ...