Most everybody I know who's ever had a motorcycle accident rolls during it. This is mostly because you've been sitting on a device that moves forward only when accurately balanced, and when it tips the wheels generally continue to roll, throwing off force in all directions.
People are lightweight and bounce a lot when separated from the heavier, rolling machine.
Also, when compared to a T rex, people on motorcycles have really really low centers of gravity so the fall OFF the motorcycle is pushed forwards by the more powerful force of the forwards momentum. The math or physical law I am attempting to describes probably exists in some nice formula, but I don't know the name of what I'm trying to describe here. Pavement gives you road rash on contact, rolling and sliding on the pavement. My accident was on gravel and dirt as the brakes fell off while driving down a one-lane paved road on a mountain, and I had a choice of going into the mouth of a ravine or off a very steep cliff. Gravel and dirt cause lots and lots of skin abrasion and make you bounce more as you hit irregularities.