Fred, are the boards too long? Are they confined in their spot and not allowed to expand and contract? I did read that they are this way all year around.
I have gone to trouble shoot some decks where several boards have bowed in the middle of the deck. after checking out the problem, I have determined that the boards were cut a bit too long and confined in their spot and not allowed to move. I shortened the boards, added new ones and the problem went away.