Don’t think it has anything to do with what car someone is driving, if they are a bell end they’re a bell end,
BMW drivers have a reputation for being total wankers, I doubt that’s true of all of them, but when you get a name etc it sticks like mud
Have a look on YouTube at the super car fails

total comedy
Oh and for the record fast fords have always had the reputation of being associated with “chavs” remember the big fluffy dice hanging from the rear view mirror and go faster stripes, it’s nothing new, in fact it’s decades old, the focus RS has fuck all to do with it, plenty of people still drive Cosworths like lunatics too and usually end up wrapping them around a lamp post or a tree
This word chav is a relatively new thing, it used to be boy racers when I first started getting into the motor vehicle scene