This might have been caused by DLM - did you copy in or still have the original catalogs for the backup sets in that USB drive present on the Backup Exec server - as if you did within 1 hour of reconfiguring the drive it will reclaim/delete any expired sets and I suspect all of the sets were expired.
Normally Backup Exec 15 marks a USB disk that has been offline for longer than a couple of weeks as Read Only just to cover for such situations - however this only happens if you leave the drive in question configured in Backup Exec and don't delete it. Note: You can also set Read Only manually in the properies of the device inside of Backup Exec.
Next the DLM expiry is in part maintained by the catalogs so if you don't have the original catalogs Backup Exec (afaik) should stick on 1 year expiry as new catalogs are generated.
Finally if you are not sure about the status of any disk containing backups sets you are about to reconnect to a Backup Exec server than you can always disable DLM using the information in this article:
http://www.veritas.com/docs/000021156
After using this article and getting the drive visible and cataloged inside of BE you could then manually set the read only status on the drive which would then allow you to re-enable DLM against other devices and you can use the retain settings to change specific expiry on Backup sets as well.