It originated on a move set (I forget which one) because the Valedictorian on the right got fed up with being pelted by the casings being ejected from the gun of the Valedictorian to his left. So the Valedictorian on the left canted his gun to the left, causing the empties to be ejected straight up.
Problem solved.
Hot brass to the face isn't fun.
A little firing on the line courtesy is nice.