The underside of your carpet has begun to separate from the top. This is a common occurrence with fairly new as well as older carpets, and especially those exposed to heavy wear or chemical action.
Your carpet is actually like a triple-decker sandwich. The topmost layer is the face yarn, which is held firmly in place because it is tufted or sewn into the middle layer or "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, can also cause embrittlement and eventual weakening of the latex.
In a few cases, your carpet cleaner may be able to remove the old latex and reglue the two backings together. However, this could be a costly procedure because it is time consuming and requires a great deal of skill.
If your carpet has begun to show backing separation, discuss it with our cleaning professional.