because you touch it.
Your joints are releasing the synovial fluid and makes a cracking noise.
Well actully, when you crack your back, its not like a bone or joint cracking. Its sort-of "bubbles" in your back from spinal fluids.
Almost immediately. If you paint each nail and watch for a few seconds you can actually see it crack.
So your body can release its junk, which is part of the human digestive system. Also known as 'pooping'.