A wall to wall carpet can get loose from a variety of reasons. If properly stretched at the time of installation it should look tight and remain that way. A carpet can get loose from furniture moving. Sliding heavy furniture back and forth across a carpet can cause it to loosen. Excersise equipment if not properly positioned may also cause this problem. Wheel chairs, carts, toys and other items that roll. Over the years a carpet can loosen. Over wetting and spills are another problem.
Basically normal wear and tear may cause a carpet to need restretching. Call a Carpet repair technician and the problem can be fixed by stretching and re-installing it back into place.
Often times a customer will say my carpet was installed wrong and talk about the installers. This may or may not be the reason. There is usually some kind of warranty that goes along with the installation and talking to the right person where the carpet was purchased can solve the problem.
Carpet stretching does not necessarily mean moving all of the furniture out of a room. Sometimes sliding the furniture from one side to the other, with furniture sliders, may be sufficient to allow proper stretching. An internet search can lead you to the right person. Check their website for the proper credentials. Years of service, good reviews and other pertinent facts. Also your local Yellow Page directory and word of mouth are another source of information.
No flooring medium is perfect. Take care of your carpet and it will last a long time.