[Growler]

The human body is an amazing thing.

Sounds a little like a nerve impulse that's triggered by your putting your body into some physiological state.

Can you replicate this reaction if you're sitting upright in a chair, rather than cross-legged on the floor? Or does this only happen then?

I imagine that if it's only when you're sitting on the deck, that it's related somehow to the nerves/muscles in your lower back or upper legs; likely some nerve or blood vessel pressure.