The part of the story that both sides tend to agree on in that the victim was not restrained by a seal belt while in a seated position.
did he became angry or belligerent enough to start crashing around and head butting the van interior where he was confined but NOT restrained.
MY POINT: The easiest place to snap or damage the spine is just below the brain stem where the spine begins. The victim COULD have UNINTENTIONALLY did this to himself.
If true... he probably went into shock and never fully recovered consciousness until he died, which I believe occurred a week after the incident.
Read up on the literature of biomechanics of cervical spine injury.
The force involved is found motor vehicle accidents, not head butts.