Motorbike may be an important factor, if the bike is taller but the bars are still low, then there may be a lot of weight on the front of the shoulders. Problems at the back of the shoulder are often caused by shortening of tissues at the front (pec minor, pec major) and a more protracted posture of the shoulder itself (rolled forward).
Generalised shortening of the anterior chain of muscles causes a poking forward of the chin with contraction of sternocleidomastoid. This is a stress or startle posture, may be associated with high adrenaline, or the flip-side, fatigue. Or simply sitting on a bike with a high seat and low bars.
Lastly, shoulder problems also often originate in a sway-back posture, which is the appearance of shortening of the latissimus dorsi. How often do we as adults hold our arms over our heads and allow this muscle to elongate? Not often in some cases. However, the role of the lats also involves balance of the pelvis, so the function of the lumbar spine and pelvis can easily be involved, and in turn, the big groups of stabilising muscles in the thighs.
In short, it's a whole-body problem, but hopefully those clues will help.