95% of the time, it's the DPS that are the issue for me.
Healer is in low gear, or is unskilled, and struggles with medium sized pulls, so I back off a bit.
Next thing I see is a dps pulling extra mobs, and the healer desperately tries to continue healing. So I switch to sword oath, /p "You pulled it, you tank it", and the extra group starts running everywhere.
Healer usually ends up with most of the aggro and I get all the blame for not holding it together and a dismissal. SMH
I especially enjoy it when I requeue into the same group and force them to abandon.