The Spirit Realm

Are you certain on what you posted yourself?
I'm pretty sure I've seen the pack of pets split up and go for lower HP targets even though the one they're beating at is still alive, not mezzed and not changing forms or anything like that.

Pets have the same aggro mechanics as mobs (they are mobs after all). They treat you are the source of their hate list.

Initially they get aggro on the mob you target them at. Any mob that affects you (spell, dot, hit etc) gets put on their aggro list.

Low HP aggro >>> all others. so they will leave primary target and gamg low hp aggro mobs

I've seen this a heck of a lot, especially when root/rotting multiple targets and throwing the pets at one of them. I used to have this happen a lot when I would solo. Nothing like having 3 mobs all DoTed at the same time for leet XP.