OOS often develops over a period of time. It is usually caused, or worsened, by particular types of work. Repeated movements, sitting in the same position, or in cramped positions, for long periods, and forceful movements are usually involved.
Stress can be a factor, too.
One theory is that OOS is caused by muscles being tensed for too long. Muscles and tendons can recover from fatigue if tasks are varied and you take regular rest breaks. Muscles and tendons may be strained beyond their ability to recover if you work without varying your tasks or taking rest breaks. To find out how it feels - shrug your shoulders. Raise them as high as possible, then let them fall - straight down, not forward.