There is a number of reasons why these accidents occur. The biggest two are. They violated the first firearm safety rule of treating all guns as if they are loaded. That rule means don't point a gun at anything you don't want to destroy, even your own body. The second reason is they had their finger on the trigger. How that finger got on their trigger can happen a number of ways.
When you grip your handgun place your trigger finger on the frame or slide. Do not rest your finger tip on the trigger guard. It's to easy for that finger to slide off and engage the trigger. There is a number of situations that can cause your trigger finger to engage the trigger. Here are some of those reasons:
Inexperience - Placing your finger on the trigger when you pick up your firearm. Often in my Introduction Course and Defensive Handgun 1.0 Course I repeatedly have to tell students to take their finger off the trigger when not shooting.
Losing your balance can cause you to grasps your hands. Finger slides off the trigger guard or frame, applies pressure on trigger and gun discharges.
Startled response - When scared by a loud noise, someone bumps into you or your surprised by someone.
Racking the slide - with your barrel pointed at your forearm or hand. The violent action of racking rapidly can cause your finger to engage the trigger.
Inter-limb confusion - an action with your support hand is mirrored with your gun hand.
Alcohol consumption - handling your firearm when intoxicated.
Jumping or single leg kicks have been identified as actions that can lead to pleasure being applied to the trigger
Violent action with your support hand, pushing someone away or pulling them in can lead to unintentional discharges.
To avoid self-inflicted injuries or shooting someone else practice safe gun handling by following the Four Firearm Safety Rules:
- Treat All Firearms As If They Are Loaded
- Finger Off Trigger Until Ready To Shoot
- Be Sure Of Your Target/Threat Foreground & Background
Be Safe & Aware