It's not just campgrounds that do this. As others have pointed out, try finding a hotel room near a venue for a major event, whether it be a football game, a baseball game, a concert, a traveling Broadway show...All events where hotels can and likely will raise prices astronomically.
I recently went to the Pepsi Center in Denver to see Maroon 5. Normally, when I go to concerts there, I book a hotel room in advance and stay overnight, as I don't like to drive late at night. I waited too late for this one, and the already ridiculous downtown Denver prices were even more ridiculous, 2 and 3 times their normal rate. Hotels closest to the event were in the $400-$600 range! I drove back home that night.