Well could still be from the water, even if you're not specifically showering the pot. I've seen this happen with certain gaiwans too. I think if the fit of the lid of the pot or gaiwan is good enough, you just get a natural vacuum seal. In fact, my friend was telling me how a friend of hers was demonstrating how he could lift one of his pots by the lid after filling it with hot water (do not try this at home, folks!).
But could it maybe also have to do with the tea's alkalinity? Given the amount of bubbles often seen with both, I think young sheng pu'er and yan cha are both probably pretty alkaline. (Has anyone else ever gotten bubbles forming across the top of the pot when they lift the lid off?)