He swallowed drugs...which broke his neck?
Ok
He swallowed drugs which caused him to go bonkers and have trouble breathing. Thus, the reason he kept banging his head on the walls of the van.
Or he swallowed drugs that made him numb to pain and not in his right mind so that he slammed himself a little too hard into a metal wall... something criminals are videoed doing all the time in the back of police cars, by the way.
Totally numb, can’t feel anything.. That’s when thugs hurt themselves. No tolerance to pain!