You are describing a training issue.
Not really, though training can help to a degree. It's a physical issue. Your body can only focus on so many things at once. I once had it demonstrated to me with a bowling ball. Stick your fingers into the holes on a bowling ball and hold it out at arms length. You can hold it a good while and have no probs. Have someone slap your face while you're holding it and it falls *every* time. I've seen countless guys all drop it, several dropped it twice even though they knew what was going to happen. Without a LOT of specific training, your brain can't process the holding of the ball and other processes at the same time and sudden physical pain overwhelms it. The same thing can be demonstrated with an old party trick. I first tried this in the mid 70's in Germany. Have the person hold a good weight out at arms length and then drop a few crystals of sugar or salt onto their tongue. They drop the weight pretty much every time. The same thing happens when you flash a bright light in their eyes. I've seen people actually wet themselves when exposed to a strobe light before. That's the same principle behind a flash bang grenade and also a strobe stun gun. It's what's happening when someone says "it's too noisy in here, I can't concentrate".
In real action, you can often see soldiers insert a mag into their weapon, pull the trigger a few times, change mags and do it again without once actually charging or firing the weapon. Bolt actions are the same....a soldier can think he's firing his weapon and at the end of the action, find a pile of unfired ammo over to his right. Shotgun shooters in competition often suffer from short stroking and double stroking for the same reason. That's why live fire exercises are so important.
You can train yourself to compensate to a degree, as in teaching yourself step by step firing sequences etc, but you can't totally do away with it. Not physically possible. There comes a time when *everyone's* brain gets overloaded and it stops processing info in the correct manner.
As I said, you can often watch the "experts" in action and still see the old slippery hand. That's why shorty weapons tend to always have hand protection on the pumps. The forearm will have a hook or a hand stop or a strap to keep the hand from being shot off by accident.