Retracting the scapula isn’t the name of the game so much as depression…
But yes, the answer to the question is as stated - usually impingement.
The shoulder and the hip are both similar joints in structure, however the hip needs to be very strong and because of this it loses a lot of mobility.
The shoulder on the other hand is the opposite, it is designed to be mobile but due to that it loses a lot of strength/integrity.
There is that, plus the structure of the rotator cuff in relation to the deltoid and the sub-acromial space… they (the muscles of the rotator cuff) are NOT trained as much as the deltoid is in standard weight training regimes, and as such the deltoid then ‘takes over’ much of the work in shoulder support from the ‘cuff’.
This is not the purpose of the deltoid and as it is such a powerful muscle (in comparison) it can pull the head of the humerus into the Glenoid Fossa (shoulder joint).
This leads to impingement in most cases.
Impingement is when a soft tissue becomes trapped between structural (or hard!) tissue… the sub-acromial bursa, supraspinatus, infraspinatus and subscapularis are the 4 soft tissues liable for shoulder impingement (the teres minor does not pass throuight the sub-acromial space).
The space in the shoulder joint is the smallest with the shoulder flexed or abducted… and if the humerus has been pulled up into the joint due to the deltoid becoming disproportionately strong, this will lead to impingement when the arm is lifted in this manner.
Also inflammation can increase the size of certain cuff muscles leading to impingement - often due to repetitive use, say weightlifters, swimmers, etc. The sub-acromial bursa is the most common for this type of injury i believe.
Focusing on the external ‘show’ muscles with little understanding of the more structural tissues will eventually lead (in most cases) to injury (‘i keep my rear delts strong’).
It is ALWAYS wise in programs where you do a lot of pressing (and generally in programs based on muscle strength and/or size and physique training) to:
- Do lateral and medial rotation work to train the rotator cuff directly.
- To do as much pulling (vert. and horiz.) as you do pressing (vert. and horiz.).