I suspect that the shooters were snipers who were already alerted by tactical radio that an officer was in peril inside the restaurant. As this U/C cop was ejected for wearing false colors, the snipers were watching with extreme stress. If they saw a Cossack with a blade out, near the U/C cop being thrown out, BANG! Head shot. First Cossack killed.
Then it’s “sympathetic firing” by the other snipers with views of the scuffle turning into a police massacre.
Cops not behind a scope would not fire, they didn’t have the view or the awareness of the unfolding situation in front of Twin Peaks. You won’t just fire into a crowd because some others are firing.
But the snipers with the clear views from above sure would.
Makes sense.