Many of these shootings also happen in places that could be considered 'car-towns'. That's what I call places where there is a sizable percentage of the population that lives out in the country. Vermont has tons of them. They're anywhere between 20K and 200K people. So with that said, I definitely see a correlation between cars and these shootings. Inner cities have drive-bys and gang violence, smaller cities have shootings. Just look at the list of school shootings and such:
Most of them happen in places that don't even reach the 200K mark. Sure, you get the occasional shooting in places like Detroit, and NYC but they're vastly outnumbered by the places that one could call 'car towns'. It might just be that cities under 500K are everywhere, it may just be pure coinkidink, but I think that probably plays a far bigger role than mental illness or even drugs.