Someone who brakes his or her car for no apparent reason. This person could be driving in front of you, or, God help you, may be the driver of the car in which you are currently driving.
TOM: (yelling out the window of his car) Hey! What are you braking for? There's nothing in front of you! Come on! Move it!
SAM: Tom, he's just a random braker, dude. His first instinct is to brake. They suck...