As Arne said, this could easily fill up a book. However, if I just pick ONE reason, out of many, that I shoot high, low, right or left, it would be:
--High: dropping my string elbow, which causes me to pull down on the back of the arrow, thus angling the arrow up and causing a high shot.
--Low: dropping my bow arm too soon after the shot, which I never think I do, but is probably why I miss most shots low.
--Right: Plucking. Almost always caused by losing back tension just prior to release.
--Left: Torqueing the bow or the string, caused by too much tension in the bow hand or string hand or arm.