It sounds like you need Auto-Failback? Under Network-->Traffic Groups -->group name. Should fail back to the default device after the unit is
Auto-Failback:
Specifies whether the traffic group fails back to the default device. When checked, causes the traffic group to float back to the default device whenever that device is as available or more available than another device in the group. This occurs even when other devices in the group are more available than the default device. When cleared, causes the traffic group to remain active on the current device until that device becomes unavailable.
Failback-Timer:
Specifies the number of seconds the system delays before failing back to the default device. The range is from 0 to 300 seconds. The default is 60. A value of 40 to 60 seconds allows for state mirroring information to be re-mirrored for traffic groups.