I've often wondered why people do that, I can't think of the number of times I've asked myself "how the hell did they get themselves in there?" I guess it must be a last ditch fight or flight response. Just as someone being asphyxiated thrashes out just before they go unconscious to try and fight of whatever is restricting their breathing, the body goes into autopilot to try and defend itself by tucking into the most defensible position. I've seen too many jobs like that and never actually heard what happened after we handed them over either to the resus unit or to the officers doing hospital guard, hopefully there is more to it than the typical outcome.