I will go ahead and tell you some factors:
1-Balance of strength between antagonist/agonist
2-Mobility/flexibility
3-Work load/underrecovery
4-The presence of postural problems such as thoracic kyphosis / winged scapula / humeral internal rotation / protracted shoulders
5-The presence of past soft tissues injuries (Trigger Point & Adhesions)
6-Every day activity and postural habits
7-Structural balance in training volume/intensity
8- Structural problems such as Leg Length Discrepancy and acromion shape
I have seen MANY therapist taking care of all those 8 factors and forget the simple sleep position. Sleeping on your side can’t be ok for your shoulders, same thing if your sleep on your stomach…