Any number of factors. Banging 808 drums that shake everyone's rear view mirrors, big rims, candy or iridescent paint jobs, garish paint jobs, hydraulic suspensions, tinted windows, musical horns, unnecessary gold trim, extremely lowered suspension, 22 inch tires, 32 inch tires, bosozuku tier fiberglass body mods, stick on logos and windshield stickers about jesus or some innercity racing gang....I'm not sure about this "driving while black" business. How are you supposed to preemptively determine someone's ethnic background while they are in a moving car. Unless they have the windows rolled down and are leaning their head outside in the middle of the day, I think it's pretty hard to make any positive identification of a random driver's physical appearance.
There's quite a list, my dear, as black fashion trends don't end at sagging pants & Jordans