When a high priority operation attempted to find an available drive, all the drives were being used. To free up a drive for this operation, the client session identified is being cancelled by the system.
The lower priority client session is cancelled to free up a mount point (drive).
When a drive again becomes available, restart the session that was cancelled. This session was most likely a backup/archive session and you may just want to let it restart automatically during its next scheduled backup window. If this message appears frequently, you may want to increase the number of drives available. See the MOUNTLIMIT parameter on the UPDATE DEVCLASS command.