[Mailman-Users] qfiles/bad is full of files...how do I find out why?

For every one of those files, there should be an 'error' log entry
saying most likely, "Skipping and preserving unparseable message:".

These most likely result from malformed spam messages. You can look at
the entries with bin/dumpdb -p.

Other possible but unlikely messages are "SHUNTING FAILED, preserving
original entry:" associated with a traceback, "Unpickling .bak
exception:" and ".bak file max count, preserving file:".

I was on my way out the door when I sent the above, so here's a bit more.

See the documentation for the settings

QRUNNER_SAVE_BAD_MESSAGES = Yes
BAD_SHUNT_STALE_AFTER = days(7)
BAD_SHUNT_ARCHIVE_DIRECTORY = None

in Defaults.py and if the setting for BAD_SHUNT_STALE_AFTER has not been
overridden in mm_cfg.py and there are files older than 7 days in
qfiles/bad, make sure that Mailman's crontab is running cron/cull_bad_shunt.