[Havoc]

>The bicycle is pulled downhill by gravity, but rotates around the brake hood

Not convinced. The bike is close to stationary after the fall, then turns a full 180 and hits the next cyclist with quite a bit of force. Notice also that the back while is in constant contact with the road as it makes that 180 degree turn. Doesn't look gravity driven to me at all...

[stouset]

As I replied to the grandparent, rotating wheels carry enough rotational inertia to spin a lightweight bike.

https://www.instagram.com/p/sh0fSPirPW/?modal=true

[gambiting]

I feel like this should be obvious to anyone who has ever fallen off a road bike. I know I have, and this is very frequently what happens. But of course if someone has no experience riding bikes and views the video it looks like cheating.