The laws of physics work every time for good or for pain; in this case for pain.
Good driving technique, mindfulness or knowledge of physics; either of these applied in this moment could have prevented this accident.
There are a number of factors contributing to this accident.
We will only discuss the physics portion here.
To perform a wheelie the bike must be made to rotate around the center of the axle in the rear wheel.
This is done by ove rthrottling and/or help from the driver in terms of shifting the drivers weight to the rear.
It will be simpler to look at the case in which only over throttling occurs.
The following is a conceptual explanation, not a detailed analysis of forces, vectors, torques etc.
The driver over throttles, this produces more torque at the sprocket than the resisting torque ((weight of bike + rider) x gravity),so the front of the bike rotates upward.
Fun so far.
The thing keeping the passenger on the bike seat is the force of gravity, friction and the angle of the seat.
When the bike is level gravity is pulling the passenger straight down.
But when the front of the bike is elevated part of the force is pulling straight down but another part is pulling down the slope of the seat.
And, the part pulling down the slope of the seat is opposed by friction.
If the force pulling down the slope of the seat is greater than the friction between a riders' bottom and the seat cover, the rider slides backward.
In this accident the driver slid backward pushing the passenger rearward and the passenger slid toward the rear and off of the seat.
Another complication was the return of the bikes front wheel to the pavement which caused a rotation around the bikes center of gravity.
This occurred at a very bad time for the passenger, just when the passenger was unbalanced on the back of the seat.
That rotation caused a little "flip" at the rear seat, flipping the passenger off while in a vulnerable position.
When doing extreme stunts it is common sense and courtesy to know all of the variables affecting the outcome.
The laws of physics operate whether we want them to or not.
Don't get on a powered vehicle with anyone you think does not value your live and health enough to find out.
Bruce Please go to the following link to view the basis for this analysis.
http://www.
gridesinc.
com/RandomStuff.
html.
View the movie clip "Outrageous Wheelie.
"