It must be the direct sun light... I doubt it would be normal day/night temp changes.
Is it all of them or just ones at the end? If at the end, then it could be how the weight of the others is the end ones against the shelf uprights.
If they warp in that heat then that heat can also be used to un-warp the record too by putting the damaged ones between plate glass in the same place - take advantage and use it as your new recording fixing area. I very very much doubt that the grooves themselves will be damaged.