Q: I hit a light post while backing in R shift and my car suddenly accelerated forward and hit another car. How is that possible?

Hi. In July 2016 I had a car accident in parking lot and got ticket for that. When I was backing my car, I hit foundation of the light post which was one feet ahead of the post (did not see it), and suddenly my car accelerated forward and hit another car in the parking lot. I explained to the police officer that I hit the post when backing, but I do not understand how my car accelerated then forward and hit other car. Could you explain if this is theoretically possible that car accelerates forward when hitting something while backing and being in R shift and what is explanation for that? It is automatic transmission. I will use this info in court to dismiss my ticket by showing that it is not necessarily my fault and some kind of electronic or mechanical malfunction is possible due to the hitting the post. Thank you very much in advance! Sincerely, Orkhan Imanov.

My car has 80000 miles.
My car has an automatic transmission.