The reason that most tanks that I see die in my experience are the attacks that aren't on a strict timer but rather a 'cooldown' that are also off the bosses' swing timer.
i.e.
Thorim: unbalancing strike. This attack not only hits hard, but it can occur at the exact same time (within say .5 seconds) as a melee swing, which could also be a crit since you are no longer crit capped and essentially "one-shot" a main tank before the off tank can even react to the main tank getting the debuff.
Gornok: Impale: once again with a ~1.5 swing timer, there is a very good chance that this attack will occur within .5 seconds of a melee swing along with rolling dot, which can come together for a tank "one-shot"
Anub: freezing slash. Once again, off the swing timer, hitting for ~15k (unable to be mitigated) and additional not on a timer but rather a cooldown, which can make timing tank cooldowns very difficult. In addition, in phase 3, this slash can occur with a melee swing for another ~20k AND a leeching swarm tick for `10-15k, all within .5 seconds. This results in a massive amount of spike damage.
The point is not that freezing slash will kill you, but since the attack is not on a timer, you can't anticpate when it will happen aside from watching the cooldown timer to come up, and it may not even use it for another 30 seconds.