I was thinking about something else a little earlier, the effects of over/under inflation on tires. The way the center of the tread comes up is what I was trying to grasp. When the sidewalls deform, they bend. So, the part of the sidewall that is on the rim cannot move (not easily at least) because it's fixed to the wheel. This means that the sidewall must bend, as the tread cannot be stretched. As a result, the tread ends up being pushed together by the sidewall, causing it to bend upwards in the center. Also, when the you increase the vertical load on the tire, I think it would then result in similar deformation to lowering the pressure.