These things happen. Hydro tractors are noted for rolling away. Of course on that steep of a hill it was absolutely clear negligence to not set the parking brake, and also set the front bucket to act as a brake. With a gear tractor on a slight slope, usually when you shut off the tractor and leave it in gear, the friction of trying to roll over the engine will prevent the tractor from rolling away. But a hydro tractor will bleed down pressure and is almost guaranteed to roll away without the brake set.
But I never rely on the parking brake either, I set the bucket on edge to dig in and also if the 3pt has something on it like my blade or ballast barrel, I set them too. The terrain here is very hilly, and you will only make that roll away mistake once. I suggest the guy with the JD was very lucky and no real damage was done to property or to life and limb. It could have been a lot worse.