To me that looks like the normal unassigning of media during the image cleanup process which runs automatically on the system.
Initially it will be based upon the retention levels you've set on your backups. Once all images on a piece of media have expired then it can be unassigned by the image cleanup process (which runs bpexpdate -deassignempty) which makes the media available for re-use.
I would double check your retention periods on your yearly & monthly backup policies. If they prove to be incorrect (or were at the time of the backup) and you are sure you have 'lost' data that you shouldn't then you'll need to re-import the data on this group of media.