Why am I getting LOG_BACKUP as log reuse wait reason? I have a few databases all in full recovery mode and with mirroring activated and I do transaction backups every 60 minutes.

When I query sys.databases a few of my databases has LOG_REUSE_WAIT = Nothing but some has LOG_BACKUP and it never seems to change.
There is nothing to be alarmed here. Even if you are taking periodic transaction log backup you will see this Log_Backup under log_reuse_wait_desc because last transaction log was not able to clear any VLF (in other word - VLF was not marked for reuse) due to low activity and not many transactions are being logged. 

Here is the precise explanation 
http://www.sqlskills.com/blogs/paul/why-is-log_reuse_wait_desc-saying-log_backup-after-doing-a-log-backup/