Determine why phones failed over to primary subscriber and then returned to primary subscriber - Call manager

Our phones failed over to our subscriber at our DR site. and then they failed back. Trying to determine the root cause. I was looking through the real time monitoring tool and looking at some traces. I don't which one I would select in real-time to show this? Is there a way to see how long a phone has been registered to a certain call manager? Where would I look to determine the cause of the failover? I've looked at the routing at both datacenters and don't see any route changes.

Any help in pointing me in the right direction would be much appreciated? Thanks