While the pecs and the lats are primarily movers of the arm, the rhomboids, along with another group of muscles known as the SITS muscles, provide stability to the shoulder joint. So as the pecs and lats have worked hard to rotate the arm inward and the rhomboids in the upper back have become weak, there is a loss of stability in the shoulder joint. Those strong pecs and lats amplify movement, while the weak rhomboids sacrifice the anchoring necessary to prevent the arm from moving beyond its range of safety. This makes for a vulnerability to shoulder injury. A good way to prevent injury is to do
strength training for the upper body, and include the rhomboids. If you pinch your shoulders together, you are using the rhomboid muscles.