The paper and foil material both expand/contract as temperature and humidity change, but at different rates. If the paper grows by 1 mm and the foil only grows by 0.1 mm, then the card will bend to keep the bonded points bonded but allow this expansion (the slight difference in radius between the inside of the curve and the outside is enough to compensate for the different card/foil lengths). This same effect is used in bimetal switches in some thermostats and thermometers. One solution would be to find the temperature and humidity at which the cards are straight; however, in practice using a hard case or binder page is much easier.
tl;dr: it's magic, and dmaz' solution works