Have you ever noticed how a group of three to five people in a public setting, if they stand around talking instead of getting a table, invariably do so next to a resource or strategic traffic area of some kind? Check it out sometime. In a fast food setting, it's clustered around the trash / tray cabinets. In a coffee shop, it's around the milk / sugar / etc area. In areas with limited traffic space, it's in a doorway. Note that in most cases, none of the people involved are actually using the resource in question - but rather than withdraw to a corner, they cluster around a resource or strategic area.
Think it's coincidence? Pay a little more attention next time a completely oblivious-to-the-world knot of people is standing around someplace functionally important. Notice how when you say "excuse me" and the person in the way turns and gives you a startled look, they instinctively move closer to the resource in question. Once they have time to think about it, they'll move aside - but the instinctive reaction, while they still have that confused "what does this person want and how do I react to it" look on their face, is to edge closer to that disposal area, cream-and-sugar-bar, or doorway - not to step aside.
This annoys me to no end. I mean, don't get me wrong, I sure as hell can't fault people for being territorial - in fact I tend to be EXTREMELY territorial - but I pick and choose what I define as my territory; I don't just sort of unconsciously claim the condiment bar or the soft drink dispenser or the doorway in the name of Jimbo, God, and the King.