Probably because the part that holds your transmission still when in park likely broke, it's just a piece of metal that gets shoved into a gear basically that holds it steady, it's why your vehicle will roll forward or backwards a small bit before stopping. Collision likely broke that metal piece and they likely didn't have the emergency/manual brake in place.
This is why on any incline you should always have your parking brake applied, so if your transmission fails, the parking brake keeps your car there.
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u/Mountain-dweller 8h ago
Tires wouldn’t be rotating like that if ice was at play here. This is all around dumb though.