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.