Thanks for your insights. I am gonna have the tank dig through his logs to see what is going on.
We have has this happen with the boss at about 20% hp so it can't possibly be the Dot tick that is doing the damage. also its not cleaves that is killing the tanks. Well let me rephrase that, the boss is not just attacking one tank and the other tank dies off in the distance. The boss is 'teleporting' instantaneously to the other thank and hitting him for 30+k damage.
Assuming it isn't just a bug, the best theory I have for why that would be happening is if your tank is taunting too early. The sequence of events is Savage Arcing Slash knocks back a tank and applies Touch. 0-3 seconds later Kephess casts Breath of the Masters. When this cast finishes all threat is wiped from the current tank, and Kephess will then go to #2 on his list. He will leap to that target and put No Escape on them and hit them for a bit.
The timing is very tight on this sequence (~14 seconds), so as the fight goes on with only two tanks, the taunt window will kind of naturally push later and later. Eventually you might find yourself taunting right after Breath, Kephess will leap to you, and you might still have a second of Touch left. That would be my guess as to what happened that would cause the tank to die. The No Escape leap is happening while the tank still has Touch.
One of the best ways to avoid this (assuming I'm right) is for the tanks to taunt as soon as the Breath of the Masters cast begins. If you taunt during a cast, Kephess will still finish the cast on his current target. This buys you a couple of extra seconds of time for your taunt to refresh, so that it will take longer for the taunt window to push to the point where you're having to taunt after the Breath is over.
As this taunt window pushes to after the Breath cast (because your taunt hasn't refreshed yet), It's vitally important that your tank not taunt Kephess before Touch has expired, since Kephess post-Breath is going to do a leap to put No Escape on someone. If you do find that the taunt window is such that Breath finishes casting but Touch is still up on the previous tank, then you just have to wait for Touch to expire before taunting... even if this means Kephess leaps to a DPS.
You can control what DPS Kephess leaps to as well by say having your sentinels deliberately not threat drop while everyone else does. Then if the tank is forced to postpone a taunt because Touch is still up (towards the end of the fight), then Kephess will leap to a DPS that has good cooldowns and can probably survive for the second or two needed before the tank is able to pick it back up again.
We actually just use three tanks on 16-man because of this slightly too-tight taunt window, and that makes it much easier to deal with. I imagine this is what you're running into on 8-man, but again, I'm just speculating without logs to look at.