There are three basic causes of a negligent discharge (ND):
Finger on trigger, Finger on trigger, and Finger on trigger
But for those that want to know the three mechanisms that cause that errant finger to pull the trigger they are:
Startle, Stumble, Sympathetic Grip
The first reason your finger may tighten on the trigger when you did not consciously will it to is due to the fight or flight response. If you have ever been startled, or (guys) snuck up and “goosed” your partner, you will recognize that when startled the natural reflex is to make a fist and bring your hands up to protect your head. If your finger is on the trigger when startled it is highly likely you will have a ND.
Stumble (Imbalance Discharge)
This is very similar to the startle response in that it is your body reacting in a protective mode. Whenever you suddenly falls, your arms automatically flay out to catch try to catch something to arrest your fall. The hands grasp at whatever they can to stop the movement. If one hand grabs then the hand holding the gun will also tend to tighten. Basically this is both the startle and the sympathetic responses caused by falling.
Sympathetic Muscle Response (inter-limb reaction)
The next reason is also part of you autonomic nervous system. We have a sympathetic reflex built into our body. It is very hard to squeeze with your whole hand without also squeezing your trigger finger. Not only that, but it is also nearly impossible to apply pressure with one hand without unconsciously tightening your grip in your other hand. This is the reason good law enforcement training teaches to always holster your firearm before going “hands on” with a suspect. If you have your dominant hand gripping your firearm, and begin to fight with your non-dominant hand you may end up squeezing both hands and causing a ND.
No matter the cause of the negligent discharge, the mitigation activity is the same.
Never point your weapon at something you don’t intend to shoot
Keep your finger off the trigger until you intend to shoot
Never go hands on with someone in an attempt to gain compliance while gripping your pistol, obviously this is for situations where lethal force is not justified, but force is needed. This really is more of a LE/Security issue, but it something an armed citizen needs to be aware of.