push to backup repo periodically fails with "not fast forward"

I have a repo on another disk that I ONLY use to backup my in-use local repo. I have a post-commit hook that does the push (git push -- all remote-repo-name)

Counting objects: 71, done.
Compressing objects: 100% (26/26), done.
Writing objects: 100% (29/29), 2.31 KiB, done.
Total 29 (delta 12), reused 0 (delta 0)
Unpacking objects: 100% (29/29), done.
To file:///Volumes/Git/pose/main
   22d7f10..0037aaf  bimonthly-frequency -> bimonthly-frequency
 ! [rejected]        master -> master (non-fast forward)
error: failed to push some refs to 'file:///Volumes/Git/pose/main'
Created commit 0037aaf: Removed assertion that made no sense.
 1 files changed, 0 insertions(+), 2 deletions(-)

I'm using git as a front-end to Subversion, but I can't figure out why this is happening.

The repo at /Volumes/Git/pose/main is NEVER pulled from or pushed to by anything other than my hook. I can't understand why any push to it would NOT be a fast-forward.

Any ideas how I can figure out what's going on?