Why do my wood floors buckle and cup in the summertime?

If you have a crawl space and the cupping is upward, then hot, moist air from the outside is entering the crawl space. The wood floors then absorb the moisture from their underlying side, making the bottom of the boards expand and the boards "cup" upwards. Installing a closed crawl space will prevent any outside hot, moist air from entering your crawl space. If this condition is caught early enough, then your floors will readjust and possibly lay flat again, preventing you from having to replace your floors.