So, this seems to be a new thing, although for the most part prior to the latest expansion I don't recall many instances where there were separate fights going on such that they remained separate fights and didn't all attack you at the same time.
So, I decide to join a fight and kill what's fighting. For some reason my HEALER companion doesn't join me but joins another fight, aggro's more mobs and dies pretty fast because he/she sucks as a fighter and apparently isn't that good at healing him/her self. The of course, I die too.
Even if there are multiple targets such that they all do gang up on me, instead of taking time to bother to heal me, my comp engages one of the mobs that I'm not fighting at that moment, but that is fighting me, gets aggro and dies (I'm looking at you Acina!) then I die because my healer sucks...even my lvl 40 Acina.
It feels like healing comps have a new independent streak and just aren't satisfied with healing so much anymore.