Imagine this alternate situation, molded onto your mindless zerg rush through packs. Healer PoV: people zone in, afk a bit, get water, headphones whatever; all of a sudden, you see that the tank dies and a stream of mobs starts wiping your dps and finally the healer.
What happened, you might ask? Well, for a lack of better wording, the idiot tank, not caring about healer's mana or level of readiness, started to chain pull 2-3 packs and tens of spawning adds, in quest greens gear (thus heavily low in avoidance/parry/block/mastery) and, wait for it, HE DIES! Do you see now the difference in approach versus mine ?
Also, you don't really seem to be thinking like a tank (which should be "my responsability is to take the big hits off of the weaker friendlies"), more like a rogue or ranged reckless class ("yay, look at me, I'm awesome, I can pull threat from the tank and die like an idiot without using misdirect or other ability in my toolkit"). Yes, for a long time now DBM shows a line in chat when it detects cooldown uses and I'm almost the only one trigerring this so I know when *(s)he* didn't..