Could be a slip of the finger if he going around some other part of the pattern other than the final portion (the portion where you're heading to the target and fire). On any range I've been on (with the exception of the now-closed strafe pits at Koon-Ni in Korea), the pilot hasn't been required to safe the master arm after each pass and arm it before each roll-in. I've seen inadverent firings occur twice at the Goldwater range, once on Range 2 and the other time on South TAC. Both were apparently just accidental squeezes of the trigger while in a turn or something else.......one of those "keep your finger off the trigger unless you're pointing at what you want to shoot". Though I'm not familiar with the particular range pattern at Warren Grove, nor the reg supps for that range; my money is on this possibility. This is one of the only things that would explain such as short burst, as would the school's location in relation to the range flight pattern.