ZS3-2 Power on/off sequence and cluster

Hi,

I have a ZS3-2. Recently, there was a power shutdown. After powering on, I realizes that the cluster is in an active-passive setup which initially it was active-active.

I have managed to do a failback of resources and the resources are managed by its preferred controller now.

Just wondering, is there any specific powering down or up sequence that may cause this behavior or is it a norm that the resources will swing to the non-preferred controller during shutdown.

Thank you