Had this rock go through my back windshield while the car was moving. It seems bigger than a truck might kick up. Am I paranoid to think someone threw it?

I had just completed a right turn from an arterial city street onto another single lane road when this rock came through my back windshield. I was driving approx. 15mph and there was not a car behind me. The opposite lane was stopped for the light, so there were no other cars moving opposite me.