I think it’s unlikely to be because of their own reflections, I’m assuming they haven’t also started attacking puddles.
IME crows will repeatedly stab their perches woodpecker style when frustrated and/or angry, no idea why – it seems such a human reaction. I suspect that what they are doing to cars is along those lines, they possibly also enjoy the noise it makes. They also are attracted to shiny objects and will repeatedly stab something if they’re not sure what it is – possibly to see how it reacts.
Crows are frighteningly intelligent creatures so normal typical-bird related explanations are not always appropriate.
Sorry none of this helps you with your problem and I can’t offer a solution, although I would be surprised if this behaviour persisted – I would expect them to soon get bored with it and find something else to occupy their very unbirdlike brains.