Anyone roller skate these days?
I looked at a nice old roller rink recently that would make your teeth chatter. The flooring was cupped pretty badly. The owner made an interesting comment: The floor moved. Or, more precisely, the floor rotated. People skate counter-clockwise. And the force exerted when they made the turns at the ends caused the floor to rotate clockwise. So think about that when you are racking out flooring and doing tongue reversals, and trying to keep the forces pushing the fasteners tighter rather than pushing them loose.
In this situation, much of the problem was related to a wet subfloor. Water from the underside was causing the cupping. And the water was also making the subflooring softer, so fasteners moved more easily. This floor system was very wet. Here's a picture of some fungus I observed under the floor. You can also see a very rusty nail in the upper left corner. We may even have rusted-out flooring fasteners.
Roller skating can be strenuous, which means you warm up nicely… which means you need to keep the place cool for people to be comfortable. And cool isn't a good situation in the warm, humid Southeast U.S. in the summer. Combine that with a big vented crawl space, and we ended up with subfloor moisture issues. The solution is easy, but I don't think the overall repair cost will be very cheap.