Who knows?
Mom once told me about a road incident in which a dude on a Harley Just APPEARED in front of her car- crossing her path. He bolted out onto the street in front of her after sneaking through some backstreet, or something. So she had no choice but to slam on the breaks to keep from hitting him. As her tires screeched he turned his face to her and flipper her off.
She said that "the finger sign" is "not from my generation, so it doesnt mean anything to me. Im no more offended by it than I would be by a baby in a high chair hitting me with a rattle, but I am baffled by it".
She opined that drivers in cars must do that on purpose - rush at him with cars and screech their tires. But I dont think that is a thing drivers do. Play chicken with their cars at him just because they dont like biker dudes. He is just a jerk who likes to defy driving conventions.
You hear about "road rage". But you dont hear about pedestrians on the sidewalk getting "sidewalk rage" and beating on each other for jaywalking. And thats probably because even if you dont speak you still communicate with other pedestrians via body language. In a car there is no body language. So you cant really read other folks and they cant read you, and they misread you, and then you misread them misreading you, and so on.