I think this was the issue.
How exactly did you hook up second PSU?
Had 1 hx850 powering 5gpu+5 risers.. second powering 2gpu +2risers + 3 cards idling @ bios screen in the second rig..
I'm leaning towards the true cause of death may or may not have been related to some *Idiot* forgetting to turn off the second psu/rig before unplugging pcie and molex connectors....
(dang psu's that run fan-less until super hot anyway.)