Who is at fault when two cars rear end each other backing out of a parking lot?

I would be interested to see how this plays out. I know my dad was backing out of a parking space slowly because he had two larger cars on either side of him and a driver was driving down the lane in the parking lot and rearended him. The other driver didnt stop for at least 20 feet so he was going pretty fast, but they still gave my dad the ticket for unsafe backing. But if both of you were backing out in a parking lot I dont know how that would play out. Did you guys call the cops? I suppose the cop could have given you both a ticket for unsafe backing??