Hunter Pence has Scheuremann’s disease. If you look at the picture that is his first picture as an Astro’s player above, he is almost side arming his throw. (That picture is from the hardball times). His condition causes stiffness of the mid and upper back and kyphosis. The condition cannot be corrected if the patient tries to use their own muscles. The back of the spine grows faster than the front and thus the bones grow like a wedge. This is why he throws so strangely.
He short arms his throws. It is amazing that he is able to throw with such strength, velocity, and accuracy with a short arm throw. If the spine does not bend as it should it makes throwing significantly more difficult.
In order to throw well from the outfield one must have good throwing mechanics most of the time. That includes full thoracic extension and he does not have that. His thoracic extension is likely limited by about 20 degrees of normal. Also his scapula (shoulder blade) and humerus must move in unison well. His shoulder blade on his ribs is likely stiff as well. The muscles called the middle trapezius , lower trapezius, serratus anterior must help the shoulder blade to rotate upwardly to have excellent throwing motion. His therapist/trainers in the past asked him to do physical therapy and strengthening exercises on his shoulder and it just hurt too much so they said back off and he has managed just fine.
The muscles above help the scapula rotate well to help in the throwing mechanics of an outfielder.
The lattisumus dorsi helps you pull your shoulder back , the deltoids help you reach your arm up in part, and the triceps help to fling the ball, the biceps help to decelerate the arm. These are just very basic observations.
Watch next time when he throws. It is amazing that somehow he has overcome a major physical impairment to throw at a professional level.