As mordecaia said...it' s the fear of missing.
It' s brought on by the in-ability to FOCUS on aiming AND on pulling the trigger...or letting the string go.
You get stuck off target...soon as your close, your mind wants to switch to shooting...and away from aiming...then you loose your ability to aim.
This is where a Back tension (or Hydrualic release) release comes in. It' s VERY had to tell when the release is going to shoot. You do NOT make a concious decision to fire the bow...ALL you do is AIM...it goes off when it wants to. When ALL you have to do is aim....it' s a LOT easier to shoot accurately.
IF you are having trouble getting stuck JUST off of the target (most of the time people get stuck, or " froze" below the target.) try this.
Let the bow down,
take the arrow off
draw and try again.
99% of the time you will find that you CAN hold the pin on target! The reason you CAn do this with the arrow OFF, is that you KNOW you are not going to fire the bow...ALL of your focus is on aiming, and none is used for shooting the bow....that' s what back tension does for you.