You could try looking for any muscle imbalance. Most bodybuilders tend to have very active Pecs but very little use of the posterior stabilisers. The over training of the pecs could lead to an anterior displacement of the shoulder because there is nothing at the back to hold it central. This would mean that as they move the shoulder doesn't move as it noramlly should and would possibly give the feeling that it might dislocate.
So if you strengthen their posterior shoulder muscles it should relocate the head of the humerus back to a more stable position. The problem that you may get from the patients is compliance to complete the exercises.