It is common for people to purposely stretch or twist their own back, so that some muscles/tendons/joints produce somewhat loud sounds similar to "cracking" or "snapping" or "clicking".
Similar sounds can also be produced by intense massage, and are typically associated with chiropractics.
I would like to study about this phenomenon (in order to know how it works, whether it's harmful or not, etc.), but I don't even know what are the words I should be looking for.
As far as I can tell, the word for the sound is Crepitus, although this not only describes "popping", but also "grating" and "crackling". While not necessarily pathological itself, many conditions actually do lead to that kind of sound.
If it's especially the "popping" that interests you, the wiki on cracking joints seems to adress this. Different processes in the synovial fluid (forming/dissolving of gas bubbles) are cited as the main reason, although the snapping of ligaments over a prominence are also mentioned.
As far as actively cracking joints being harmful, several studies (see wiki link above) indicate that popping of finger joints, at least, do not produce any problems.
