How does this injury happen?

Elbow dislocations occur when a person falls onto there hand. When the hand hits the ground the force is sent straight to the elbow. there is usually a turning motion that occurs during the fall. That can push and cause the elbow to go out of its socket. For example the picture to the right. Elbow dislocations also are very common in car accidents. For instance the passenger will reach forward to have less of an impact. The force caused by the impact is what is sent through the arm to the elbow causing your elbow to dislocate, just like in a fall.