This is the default behavior of NetBackup.
If you don't want your tapes ejected every time NetBackup is through with
a drive, then you need to add the -nsu flag to ltid. This stands for "no
standalone unload." Edit the startup script that you use to look
this this:
ltid -nsu
you can add DO_NOT_EJECT_STANDALONE to the openv/volmgr/vm.conf.
works in version 5.1, not sure if in older versions.