Ever seen the result of a pre-detonation? That looks more like it was sandblasted, this is not. And the moment has nothing to do with the opening of the exhaust.
Normally you get the ignition just before TDC and because there is a slow reaction the combustion after TDC. With pre detonation the combustion will start just before or on TDC making a uge pressure.

This looks like dirts from somewhere or I would say a to low head clearance.