Everyone provided plenty of good feedback and explanation so I won't add more here regarding what has already been adressed (since there's nothing to add anyway)
But I will advice you regarding threat generation as a healer.
Many inexperienced healer tend to overheal drastically, however overheal creates as much enmity as proper heal. For instance, it is not rare to see a WHM/DiurnAST do Medica2/HeliosAspect before pull applying the hefty HoT on the entire party. Unfortunately this HoT will do nothing but make you climb up the aggro ladder.
I don't know about your healing skill/ playstyle but I assume you're a rather inexperienced healer so these are the kind of things you might be doing (mind you, a lot of people did, I did!). So perhaps also be mindful of that.
Finally, by all mean if you consider that the tank takes too much damage due to poor CD management feel free to tell them.
Just this week I did OS5 and the tank kept being nearly one shot by the TB and he actually died twice from it. Also DPS also ripped the aggro a few times
I told him if he intended to stay MT that he should actually stay in Tank stance because he clearly didn't know what he was doing and that he putted the group in jeopardy by dying and not making enough threat.