I think it has to do with seeing places normally filled with life and people empty. Sort of like being in a school or an office after hours, or why abandoned theme parks and carnivals are so unsettling. You walk around these maps on guard for a pyro lurking around the corner, but he's never there. You are reminded of the countless hours you and your friends spent chasing each other through these corridors, and now you're the only one left. When you leave, will this place still remain?