The first time I did this, I wasn't sure if speed might have been an issue. i wasn't going very fast, but I was moving. This last time, I know exactly where and when it happened. There was only one rock, about twice the size of a basketball, but shapped like a footbal. It had a scuff mark on a small sharp spot (the pointy end of the football) from the impact with the rim.

Sal - I watched your video and I see what you are talking about. My circumstances were a little different. This was the front tire, and it was turned all the way to side at the time. I am speculating this sideways force is what pulls the tire sidewall over enough to expose the rim. I wonder if airing down more as you suggest would make that pulling sideways of the rubber off center from the rim worse, or better? My thought was a stiffer tire (more psi) should stay more true to form right over the rim.