hi
i agree with the suggestions of casuses
overtraining of pecs,lats
ignoring og rotator and scapular stabilizers
theory of following path of least resistance by humeral head as described by shirley shermann
other factors that might be contibuting will be lack of proper warm up and cool down phases,lack of stretching of the muscles,general mobility exercises
improper body mechanics while doing weight training,for eg latts pull behind the head is shown to cause trauma to cervical structures.latts pull down toward chest is suggested as an alternative