I have had the same problem and have determined the backup stgpool will
start a separate process for each node you are backing up (Up to
MAXPROCESS).

I don't like it since I have 1 node that is an oracle database between 120
Gig and 150 Gig.

I ended up (eventually) running the backup stgp more often, even while
backups are still running.

I also swapped my offsite tapes to our faster DLT7000's in our STK Library
and I now have breathing room in the offsite process.

Bill