Thanks for the ideas.
There's no ER running on the system, so that couldn't have been causing
it.
There were some onmode -z commands run, so that may have caused it, but
I'm not sure.
After the initial "sighting", I saw the same situation on a different
server, on a different machine. I noticed that after some time, the
locks on both machines went away. There was nothing in the logs to
indicate any problems.