My opinion. During the tussle, an agent removed his firearm, upon bringing it up and away from those present, for some reason it discharged. That Sig is known for accidental discharge, or the agent accidentally fired the weapon in the heat of the moment. That negligent discharge is part of what started the rest of the shooting.
Also, it's possible when the agent disarmed the man, in the chaos, another agent saw the weapon and not realizing who held it, yelled "gun". That, in conjunction with the negligent discharge, set the ball rolling.
My opinion. Had the armed man not inserted himself in to the situation he'd be alive today.