You have super lax joints. Some people have this. I do.
I have recently seen a doctor and a physio about this.
It is perfectly OK. They said if you keep doing this you might get arthitis when you're 50 (I'm 22). They said MIGHT. They had no idea. They said if it is not painful then there is not a lot they can help me with.
I find it relieving to crack my elbows, knees, spine, knuckles, neck and shoulders. Scares the shit out of people when I do it!
I'll see in another 10 years if I have joint problems....
Never hyper extend when doing lifts of course!
See http://www.T-Nation.com/tmagnum/readTopic.do?id=1733419 where I just spoke about this 5 mins ago.