I think the implication is that after you pull the first few trash packs (which are almost unfailingly just dudes standing around doing their normal dude jobs), word gets sent ahead. It's not like the boss has been idling around in their room the whole time, so much as they know that you're coming. They move into what is generally a trapped room that in some way gives them a home field advantage. They're taking part in the defense of their home.
I think HFC is a rather good example of this in most places. The first few pulls are just low level peons and engineers and stuff working in the siege yard. As you tear through them the army starts amassing and fighting back-- Assault. Whatsherface flees inside and brings out the big guns, and some tougher elite packs prepare to hold the hallway behind her while she tries to deal with you with the Iron Reaver. By the time you head to council, they're setting up ambushes (the stealth packs). The path to Kilrogg is lined with people who are very clearly waiting for you, weapons drawn.
You have to have a degree of suspension of disbelief for this to work but I think it more or less works. Most of the cases, the boss isn't just idling in the room you find them in 24/7-- but they know there's an army on the way and they moved into this 'arena' to intercept you-- or otherwise, you are wandering into their place of 'work' (Thogar, Beast Lord, Flamebender come to mind).