Why do npcs finish the bosses?

So, why do npcs finish the bosses? Act 2 and Act 4 bosses getting finished by npcs is really weird to me. Atleast in act 2 Asala is there from the start, even still I found it kind of weird that you beat the guy up and she swoops in last second and does a Mortal Kombat fatality.
As a one time thing I didn't really think it was worth mentioning or making a post but now that another act boss gets sniped by an NPC I have to ask why?
Did anyone else find it odd?