LOG_BACKUP wait type for Transaction log file of a simple recovery model database
Are you running SQL Server 2012?  Is your model database in SIMPLE recovery model?  Are you a bit behind with your patching?

This appears to have been the issue for a previous ask of this question, at SQL Server 2012 Simple Recovery Model with LOG_BACKUP log_reuse_wait_desc

Fixed in KB2830400  which forms part of SQL Server 2012 CU7 / SQL Server 2012 SP1 CU 4