What is your vCNS/vShield version? This was a bug at one point where Appliances would try and move/power on to a different host, but that was fixed.
What could be happening is if had the previously mentioned bug and you are using Affinity rules to lock the appliances to the host, it would cause that issue.
You should not be using manual affinity rules for the vShield App or Edge (vShield Manager will automatically create anti-affinity rules for Edges).