Fatty liver disease! So it does sound like obesity can be a problem. Was it a male ball python? And were they any signs that this particular animal was clearly overweight, or was it really out of the blue?
But yeah, I'm slow-growing my snakes, too. Not so much intentionally---they just seem to grow a bit slower when you keep them on a strict f/t diet. And if I run out of f/t on a given day, I'll tend to favor the girls over the boys.