You have a flag. Someone else wants that flag. They're coming for it. Are you going to abandon that flag? Hell, no. You're going to guard that thing. This is called camping.
But is camping only as old as online shooters? What if I told you its origins go back further -- back to the golden age of arcades [via Halolz]:
Camping is a legitimate strategy. The problem occurs when a game's levels or systems aren't designed to give the opposing side a way to deal with a camping player. Several MW2 maps come to mind.
I still love this old Penn Arcade comic on camping: http://www.penny-arcade.com/comic/2002/4/22/
I think Donkey Kong was the original camper. Just stayed up there throwing barrels like a pussy.