This is the cause of popping in your knuckles

It's called cavitation. When there are localized low pressure areas in a liquid some of the liquid flash boils and creates a bubble. (Liquids boil at lower temperatures under low pressure, that's why there are different cooking instructions for high altitudes where the air pressure is lower.) In the case of synovial fluid it's more likely to be dissolved gases coming out of solution rapidly (as a bubble) than the fluid itself boiling, but it's still considered cavitation.
Incidentally, synovial fluid (the stuff human joints are filled with) is the best liquid lubricant known to man (lowest coefficient of friction).

Still doesn't cause arthritis

I learned this in massage school and when I try to explain this to people, they look at me like I have a 3rd eye.

I always thought it was nitrogen gas. :)

*Looks at this* *Cracks Knuckles*

Hehe I can make explosions with my fingers..