Muzzle Brakes are designed to direct the gases out of the end of the barrel to control recoil.
That means that gasses are directed in a different direction than the bullet. In this case the gasses were directed to the side which just so happens to be the location of the car mirror. After the first few shots you can see the mirror moving. Eventually the muzzle blast shattered the side mirror glass and left On The Road shooters with a valuable lesson.