UV Ray Damage
Look at the picture and noticed the difference in the color of the back of the carpet and the carpet padding. Where there was a water bed for many years and carpet seaming tape that covered the padding the padding is lighter in color newer and more pliable. Where the padding was not covered by these items it was exposed to environmental factors including UV light and other environmental factors. Which allowed certain degradation to take place changing color and also the texture and pliability of the materials. Given time UV rays and environmental factors can affect the backing of carpet carpet padding and other materials below the carpet more than you may think.