AK Change Clarification Question

What triggers the door opening in AK before each boss?

I"m just curious if it's the mobs right in front of the door or some other part / kill of the dungeon (I know the first boss it seems to be the group of followers right out front but I'm not sure).

Going through it, I'm wondering, if it's just tied to those mobs, a pseudo-speed run might still be applicable (You just obviously won't be able to make it to the boss, but I can't see why looking at the areas you can't go 3/4 of the way in each section)

It's the mobs directly in front of the door.