If the board warped because it was sitting on a surface which had a higher moisture content than the wood (maybe concrete), the warpage would be due to an uneven moisture absorbtion by its two faces. Or possibly the surface it was sitting on absorbed moisture from the wood and dried one side more than the other (causing warpage). It may straighten out as the moisture content equilizes throughout the board.

I'm not an authority on this subject, these are just some thoughts to consider.