Lower back pain when standing for extended periods of time. what can be causing this?

Most people will get backache from standing for a long time. If feasible, take small breaks and sit down in between. Using a padded, soft surface where you stand will work. You work should have ergonomic standards and you may request an ergonomic assessment of your work environment. You could call OSHA (occupational health and safety) office for assistance. Wish you good health!

Prolonged standing and walking can produce pain with sacroiliitis. You might wish to discuss this with your physician to see if this is applicable in your case.

Recommend X-rays of lumbar spine to rule out structural problems, and films of S-I joints just to be complete. Physical therapy may help immensely, but would consider wearing, as needed, a sacroiliac belt if there is anticipated lengthy standing. Stretching and exercise would be beneficial over time.

One possible cause is biomechanical one leg longer than the other leg or a pelvic rotation can cause these type of symptoms