Why are master backups running at unscheduled times?
Unscheduled master backups are running
The system synthesizes master and differential backups if you are using an incremental forever backup strategy. Because the schedule runs only incrementals after the first successful master, the system synthesizes both masters and differentials from the incrementals to improve restore speed. If synthetics were not created, the restore process would have to roll up all incrementals that occurred up to the restore point before performing the restore.
To verify that the master has been synthesized by the system, check for Synthesized true in the backup details as described on the Backup Information page.
For more on synthesis, see When are synthetic backups created?