Ligaments. The two most common causes of recurrent shoulder dislocations are a generalized ligamentous laxity that causes instability or a specific injury to a structure in the shoulder that subsequently make the shoulder unstable.
Injury or congenital. There are 2 broad categories of shoulder instability. Some patients have a traumatic injury that leads to a shoulder dislocation or subluxation and sometimes recurrent instability. Other patients are born with "loose" shoulders (and often other loose joints) and may have shoulder dislocations without significant trauma. Without knowing a little more history it would be difficult to determine for u.