VMM Agent update makes VMs go down and restart on other VMHost

Hi all,

I have had a serious issue upgrading VMM 2012 R2 from Rollup 3 to 4. When I got to updating the VMM agent on each VMHost, I ran into a symptom where multiple VMs stopped running on a host, and started up again on another. I don't know what caused this. After the "Update agent"-jobs on the VMHosts, we suddenly saw "Discover Moved Virtual Machine"-jobs passing by in the Job History of VMM. We also had 1 of the 32 hosts which crashed completely and rebooted by itself.

Two questions:

1. When updating the VMM agent on a VMHost, is it best practice to put it in maintenance mode, or is it generally accepted to have VMs running on it? I can't find any recommendation in guides like http://technet.microsoft.com/en-us/library/cc917969.aspx
2. Why could VMM (or the failover cluster) suddenly decide to start the machine up on another machine?