Apologies for the delayed response. The AG backup will run on secondary for transaction log backup or copy only full backup by default, if you are attempting to run full or differential backup these will run on the primary replica, as these backup types are not supported on secondary in SQL Server.
This is mentioned in BOL: http://documentation.commvault.com/commvault/v11_sp18/article?p=18295.htm
- Full and differential backups always run on the primary replica.
- Full copy only and transaction log backups run either on the primary replica, if the primary replica is set as the preferred replica for running backups, or on the secondary replica with the highest backup priority.
The backup preference here can also not be controlled by Commvault, so if customer is seeing backup run on primary replica, they have not set the AG to use "Secondary only" preference in SQL Server. Note once again, that if the goal here is to run all backup on secondary then the full backup must be scheduled to use the copy-only advanced option.