This was wiping us a lot. Everything is fine, the run is going perfect, doing everything right, the tank swaps are clean, no one pulls aggro, then bam, lightning spears inside the defense system on stormcaller, WHILE the tank is still kiting spears around.
It just randomly stopped happening for us from time to time. We didn't change anything and just had to hope that random spears didnt wipe the group inside stormcaller's defensive system.
Read my post. Watch for it on your next run.