1. Jerking / staging the trigger
2. Staying up too long on a shot and snatching the trigger
3. Gripping too hard with their firing hand
4. Not concentrating on their front site.
5 No 2nd Site Picture / recovery
6. Sight alignment (sight too high or too low in rear sight)
7. trigger position
Wow. I've been shooting for..er..decades and I still have these problems.