Your carpet is actually like a triple-decker sandwich. The top most layer
is the face yarn which is held firmly in place because it is tufted or
sewn into the middle layer of "primary" backing. The third layer
of fabric, which rests against the floor, is called the "secondary"
backing. The two backings are "glued" together by a thin layer
of latex, a type of rubber cement.
Like most other rubber articles, latex will deteriorate with age. It becomes
brittle and crumbly and loses the ability to hold the two backing layers
firmly together. Heavy traffic, heat as well as spills of various sorts
are still other causes for the embrittlement and eventual weakening of
the latex.