A list gets locked when a list owner issues a GET command with the intent
to update it. This is because the list owner is going to send a
replacement copy. If LISTSERV processed your request, it would be undone
when the replacement list is received. Sometimes people forget to
actually update the list, and it remains locked. Some owners (mostly for
closed, internal lists) don't want the list to change, and leave it
permanently locked. A locked list can still receive and distribute mail,
it just can't be updated.
Eric