Is it a theory, I thought it was exactly what causes it. The stretching depressurizes the liquidy thing in the joints and that causes dissolved gases to bubble out of solution in nucleation points. It's not really gas build up, but more like opening a can of coke, the pressure drop causes the equilibrium to shift and brings dissolved gases out of solution. At least that's what I thought it was.

I am not doubting the theory of the gas getting out of the liquid.

I was just referring to the "urge" to crack my fingers. I thought, maybe this instant relaxation (with additional auditive feedback) results in an addiction which creates or increases my urge to crack.