That tempered safety glass is designed to really spread the impact, so I'm thinking you might not nessesaraly see a centre where it was hit if the impact was from a wider object. I'm thinking maybe someone falling or being pushed backwards into your vehicle, maybe hitting flat on their back against the glass, perhaps someone falling of their bike and going shoulder and helmet into the glass.

I had a scary night a couple of months ago where a friend and I were sitting in my car in Kings Cross (potentially nasty area of Sydney, lots of drugs and prostitution) and a pretty spectacular fight broke out outside a strip club, that began with some guys trying to bash a transvestite. I had a girl in the car with me, so opening the door was not an option as it would have brought us into the situation, and I couldn't drive out of there because there was a man being kicked half to death on the road in front of me.

There was a scary moment where this guy was actually up against the rear window of my car being punched, and I was visualising that with your broken rear window. I'm thinking someone's body weight being heaved somehow against the car.
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