What is the trigger in "The Sniper?"
I am assuming here that by "trigger" you mean the initiating event that begins the rising action of the plot and leads to the climax and resolution. If so, then I would argue that the trigger is the appearance of the old woman in the street below the rooftop where the sniper is perched. Her speaking with the soldier in the car and her pointing to the rooftop suggests to the sniper and to use that she is revealing his presence and position. Obviously, this is of great risk to the narrator as this information almost certainly means his capture or death. Therefore, he must act to remove this risk by killing the old woman and the soldier, thereby exposing himself to gunfire from the sniper on the opposing rooftop across the street. If the old woman had never appeared in the street, the sniper would never have had to fire upon her and never would have drawn fire from the opposing rooftop. In other words, had it not been for the old woman, the plot of the story would not have transpired as it did.