it could be that the guy took advantage of you. you see, the fact that you hit him from behind obviously puts you in a tough spot.
but i think, the main problem is because he claimed to have injuries.
under normal circumstances, like the point you described [when nobody is injured], police will not investigate. however, if the guy decides to go seek medical treatment for any 'pain', even if it is days after the day of accident, the non-injury accident becomes an accident with injury. in this case, all he needs to do is lodge an accident report and police will investigate. i believe this is what happened, and how you got penalised by the police. whether or not he is really injured is another matter altogether, for it is up to the doctor to decide if any injuries he sustained subsequently is a direct result of the accident. i know for a fact because some people will die die claim to the doctor that they feel 'pain' even when there is nothing, just so that they can fulfil the criteria to lodge an accident report. worst thing is, they usually get what they want.

about the claims made against you... well, you're really unfortunate. this guy sounds like a total scumbag: get you into trouble and now wants to fleece you. i think really no choice. you might have to contest his claim in court.
all the best. hopefully the case turns out in your favour. do keep us all posted. =)