First off ... you basically died due to aggro mechanics and the fact that a lot of people don't know about them.
The thread generated by healing is distributed among all opponents in fight, whereas thread from damage is only applied to the npc(s) that actually took it. Taunts guaranttee aggro for a few seconds and after those the normal thread table decides who is the next target.
If a mob group is is relatively large chances are high that a few mobs neither have been taunted nor have taken damage.
So even with the slightest heal you do, you're the the lonely inhabitant of their thread list.
In a well prepared pull this should not be a problem and you would rarely have the aggro of even a single mob.
Problem is, preparatiion of pulls in GF runs is often ... lets say ... suboptimal
When running with a good group you should experince far less problems.
This begins with knowing what to CC while the tank opens the fight. Out of stealth CCs will of course be applied before that.
It continues with the Tank pulling the aggro in a most optimal way using his AOE taunt and attacks.
The DPS should know what to take down first. At the beginning these are usually the taunted normal mobs.
This should only leave a few remaining threads to control.
Of course there are certain bosses that have random aggro, aggro resets, random attacks...