BOS keep getting their guns blown out of their hands, I've noticed it happens excessively to them, most likely because their weapons are in such poor condition.
Regardless, I'm pretty surprised at the result, I thought the BOS after patch would have destroyed the NCR vets.
Also worth mentioning that energy weapons are prone to low health while being rather clunky in design, so they make for big targets.
Botht the Gauss rifle and the Tri-beam laser don't exceed 100 item HP and can be quite bulky, whereas the Brush gun has 150 and is slender by comparison.