He has been running the 661 & the 617 successfully at 8.4v for a few years now.
Yeah, BlueBird doesn't rate them above 6.0v...
I personally don't run mine above 6.0v.

So what may be the cause is the driver cir of the servo back fed 8.4+/- on the signal wire????

The other theory in my mind was back EMF from the cogging, but I didn't see what came first, the runaway/cog on startup or a rev-up & loose sensor wire that caused a cog & then the runaway.

Have to ohm out the toasted servo board & see if there is a fused connection...

We have had great success with those particular servos for a few years & this has been the first catastrophic failure/fireball.