Why aren't they attacking?

I decided to try out the Vermin W/Mo farming build at the Troll Cave. Didn't go as well as Vermin farming, I did kill a a whole bunch of them and no drops.

But I am wondering as to why are there trolls just standing around doing nothing even though they are in the aggro bubble and everything?

An update a while back made it so that if you are dealing AoE damage to Melee enemies in groups larger than three, all but three will split and they will take turns attacking/regenerating.

Because of the Nightfall update that made them run and heal when they drop below ~50% health.

yes this update made farming that when groups are clumped easier(which is most of the very effective builds) much much harder

I thought the update only affected when doing AoE damage, didn't know about the healing part.

Because Cyclone Axe attacks all adjacent foes it is considered an AoE attack and thus causes all monsters, excluding the primary 3 being attacked, to flee