It is because the acting like an asshole implies social dominance in a highly visible manner. Power in a social setting is determined by who you can/can't ignore and who can't/can ignore you. That which you can't ignore controls you, that which can't ignore you, you control. Assholes can be notoriously difficult to ignore and also tend to be very good at ignoring/steamrolling others. Hence they appear to have social dominance.