I tried catting /proc/mounts as well and I discovered this nested mount
under proc in the chroot:
none on /var/lib/mock/rhel5-unstable-x86_64/root/proc/sys/fs/binfmt_misc
type binfmt_misc (rw)
Adding it to umountCmds fixed my issue.
This doesn't seem to get mounted explicitly by mock, hence I think it's
something happening inside the chroot that triggers its mount? I'm not
really an expert in rhel/fedora internals, any clue?