Shadow Army and Undead in FoW

The Undead and the Shadow Army in FoW attack each other, right? I remember using this to get past them easily when they spawn near each other. If I'm wrong in saying this, tell me and ignore the rest.

If I'm right, then why do they attack each other? Undead are ruled by The Undead Lich, and the Shadow Army is ruled by Menzies. The Lich and Menizies are both allies of Abaddon, so shouldn't they be allies of each other and therefore, their armies not oppose each other?

Thanks for any replies.

What makes you think the Lich rules all undead? Only the ones from Orr as far as I can see...

Yeah the Lich doesnt rule all undead. The undead in FoW appear to be masterless.

I think Balthazar is their ''master''

Eternals have Balthazar as their master and on The Battlefield there are 3 eternals that are trapped by Undead (once the Undead appear they seem to be released) Undead can be masterless as seen in the Desolation (Jokos Undead vs Masterless Undead)