Amazing as it sounds, I suspect that these guards need more training with their weapons. Glocks are handled just like double action revolvers. Keep your finger off of the trigger until you are ready to shoot. It is almost impossible to fire a Glock if you keep your finger off of the trigger.
I bet the guard had his finger inside the trigger guard.
Yeah, he effed up. Have a CZ 97, treat it like a 1911 and you stay safe.