Monsters attack party members when I have aggro

[Niwashi]

There are at least three bosses that attack completely randomly, between every attack deciding who to go to (the earliest is in Quarn's temple, the golem, after he was "broken" the first time).

And even before that, you'll see the more common pattern of bosses who have a specific attack that's programmed to go to a random party member, like in Brayflox, where Hellbender will periodically trap a random player in a bubble and later Aiatar will spit poison at random members, though both those bosses direct all the rest of their attacks towards the player with primary aggro.

It's a way of ensuring that the fight mechanics are something for all players to deal with, not just for tanks alone.

[kikix12]

And even before that, you'll see the more common pattern of bosses who have a specific attack that's programmed to go to a random party member(...)

Except tanks. It cannot be called fully "random" if it excludes the person that is top of the list.

Though I don't know whether it excludes "tanks", as in the role, or "tanks" as in the person with highest enmity. Probably highest enmity...but yeah. There are attacks that go or don't go after specific roles.