[HayZee518]

wood is a good and odd construction material. the cells that make up the pith part can be open or tight. the more open it is makes it more prone to absorbing moisture. a closer grained wood ( the grain is actually the growth rings) absorbs less moisture than the spaces in between. if when wood expands and itself is held tightly in place the expansion has to go somewhere, in this case upward or downward, again this depends on how it was installed. they say to install wood so the "cupping" [ the bowed part of a plank or beam] is upwards. your cupped board is growing upwards because of moisture. I'll bet in winter when the humidity is low that the bulge goes down.