At a guess …
Jupiter is Big. I mean, really big. And not big like a Cadillac is big. Really, actually big. That means that the higher you go in Jupiter’s atmosphere the further and faster particles up there have to go to complete an orbit. In the upper layers the circumference is really large, meaning things have to really honk along to get around in a day.
Also, Jupiter’s atmosphere is really gritty. There is lots of “stuff” in there.
So, lots of quickly moving particles means lots or reasonably high energy collisions, each of which converts kinetic energy into thermal energy. The net result of millions and billions of such collisions in a temp of 725°C?
/guess
^ Also; friction generated between different layers moving at different speeds. But I would think that the density of the atmosphere is still key.
