At your level, I don't think it's AOE from the DPS, though that's one culprit. Healing (including overhealing) does create noticeable enmity. The modifiers aren't nearly as bad (
See these charts if you want to really understand it), but if all you use on secondary targets is Overpower, it's not hard for healing enmity to overcome that. Especially since overhealing does generate enmity.
It's generally pretty easy to tell if the DPS are attacking the wrong target. Its health will drop pretty quickly compared to the sliver that Overpower does. But that's why marking is encouraged so strongly. From what I've seen, DPS who don't follow marks are the exception rather than the rule.