The human shoulder is primarily a ball-in-socket joint. The head of your upper arm bone fits loosely into a rounded socket in your shoulder blade. A patchwork of muscles and tendons called the rotator cuff keeps your arm bone centered in the socket.
Throwing a ball overhead places extreme stress on the shoulder to remain stable. It is one of the most violent actions you can inflict on any joint in the body. To accelerate the ball to top velocity, a person uses their entire torso in a kinetic chain to generate a high level of energy that ends up concentrated on the arm, launching the shoulder forward with tremendous force.