It really sounds as if you have very weak scapula stabilizing muscles, such as serratus, traps, latissimus, and possibly even some trees or rhomboids. Remember all the major RTC muscles attach to the scapula. You can work these in open chain or closed chain and Barry the angle and the speed you work on them. Also remember that a lot of evidence these days is showing the benefit of eccentric strengthening. You can look these up or we can "talk" about them together here. It is thought that RztC years in baseball are due to eccentric overload as the force in the throw occurs. The RTC is a complex of 4 major separate muscle pulley or lever systems whose resultant is to maintain a constant center of rotation regardless of where the arm is positioned. The shoulder blade is the stabilizing platform that starts in allowing this to happen. With injury things don't position or move the way they are supposed to when they are supposed to. The overall pulley system becomes skewed or biased and some muscles work harder and as the scapula has excessive movement , contact begins to happen in places it is not meant to.
The specifics along with the overall picture are important to understand.
I am using text so really won't know how well my thoughts are transcribed.