Dave,
Most often this is caused by the backup job or the storage location.
The backup job can 'forget' it needs to clean up the old backup jobs. When this happens, it is recommended to delete the old backup job and create a new job. The old backups will need to be deleted manually once you have a current backup created by the new job.
The storage location can cause this behavior if it is disconnected or loses connection. Most often this is seen with External USB/Firewire drives, but has occurred with SAN and NAS storage locations. With SAN or NAS drives, if the Drive Volume signature, Disk Signature, or IP changes, the storage appears to be a different drive and all previous images are not recognized as part of the same backup job. The fix is the same as above (recommended). Or delete all backup images and allow the backup job to create the first Baseline new again.
Regards,
Rich L