You would need to conduct a seance since only Oswald knows for sure. A couple of possible reasons though. First, he probably didn't want the SS facing him. Second, his target was JFK. To hit him from the front there was a windshield and two rows of people between him and JFK. He has a clear shot at JFK from behind. Third, Oswald was a sneaky coward. Back shooting fit his mentality.
First..supposedly left $$ and ring behind ...he was ready for the end...supposedly. Hell shoot the bodyguards they were sitting ducks too.
second...there was no windshield in the way silly. What windshield from 60 ft up?
A perfectly clear shot right between the eyes.
Grip the reality for once.
Third...how would you know third? Were you there?
Oh --you must have held a seance while you were sitting in the restroom