You probably have the job set up to "Eject Media" upon completion. Backup Exec then shows you an alert, asking to remove the tape.
You can have Backup Exec automatically answer OK to that alert. Go to Tools/Alert Categories. Select Media Remove and set it to automatically respond after 1 minute.
http://seer.entsupport.symantec.com/docs/192055.htm has some information, but is intended for previous versions. The screenshots near the end may be useful.