You seem to be assuming the tank in the story was competent. Try to think about what you'd have to do to be that squishy.
Most likely they weren't positioning correctly and getting hit in the flank/rear. While simultaneously not using any (defensive) cooldowns -- I'd bet they had Berserk and Maim running. Not to mention getting hit by multiple aoes. It wouldn't surprise me if they weren't in Deliverance either. Finally, there is a strong likelihood they were wearing broken gear.
It really takes some effort to die to two trash mobs as a tank in under two cooldowns.
