The shoulder is the most mobile joint in the body.
If you cant get the arm fully overhead properly, your body will find a way to do so. The picture shoulders a commun compensation for the lack of mobility and core balanced.
Repeated too many times, it becomes a ticking bomb, your low back becomes very sensitive to extension and developp low back pain.
You have to keep the ribs tuck down, the pelvic neutral, abs engaged, and get balanced extension from both the thoracic and lumbar spine.
Now that you know what the problem is , make the shoulder work right and the low back stops getting jammed every time you go overhead.
It calms down and low back pain goes away!
Elite Fitness Personal Trainer in Cannes & Monaco on the French Riviera.