That was explained to me by a chiropractor.
It has something to do with a build up of something, I'm thinking oxygen but my memory is a bit vague.
Anyway something builds up naturally in the joints and when it is moved quickly it releases.
It takes a while to build back up again.
Thats why you can crack your knuckles and can't do it again right away.
Ask this on the "Ask Doc Sutter forum".
You should get a more thorough answer.
He's a Chiropractor.