when is weight distribution hitch required

I'm not sure how you arrived at that conclusion, but my thinking was quite contrary. A 250 is a great tow vehicle and much more capable than my lowly 1/2 ton. However, the capability of the truck did not prevent the accident. It's possible that the driver was distracted or there was some mechanical failure, but it appears more like a textbook victim of the bow wave effect. All of the signs are there.