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.
Generic life processes? Everything is dissolved you know, including the oxygen and carbon dioxide from respiration. Nitrogen too, though it's not really very useful. The fact that it's all dissolved is how divers get the bends. As the pressure lowers gases drop out of solution and form bubbles, clogging arteries.
My uneducated guess would be that the pressure is the natural state because of the fluid/gas involved, much the same way sediment eventually settles at the bottom of a bottle of orange juice. Over time, the gas will just naturally form inside your joints that way.
