Something that’s tagged by another player should not be randomly attacking us
This normally happens when there’s a group of mobs that are ‘together’. And as far as I know this is sorta how it happened.
Like if I pull a group of gnolls and heal myself I get on the Aggro table even if someone pull a separate one from the same group.
I remember this happening in Vanilla, but I feel like I thought it was proximity threat. If you pull one mob, and it drags three more with them, those three can still proximity aggro to someone.
I could be wrong, and it could be healing yourself or doing other stuff, though. I just thought it was because the ones who aggro onto that other guy are technically only leashed with the one he hit, and still have open tables.
It’s a train 
.
If you’re fighting a mob and I hit it we now share an aggro table.
If I start hitting my own mobs before you drop combat we are still connected. If I die they will all run to you.
Atleast that’s how I understand it.
Maybe it is some kind of proximity threat. I’ve gotten aggro from mobs other people pulled that I didn’t touch at all. I’ve also had instances where a player will have a mob chasing after them and instead of it resetting and running back it will aggro onto me.
I actually like that mechanic. The world is a dangerous place, be careful.