Sacroiliitis: This is the most common cause of sacroiliac pain, often due to inflammatory conditions such as ankylosing spondylitis, psoriatic arthritis, or reactive arthritis. The inflammation of the sacroiliac joint leads to pain and stiffness, particularly in the lower back and buttocks.

Sacroiliac Joint Dysfunction: This condition occurs when the sacroiliac joint is not moving properly, leading to pain and stiffness. It can be caused by various factors, including trauma, pregnancy, or degenerative changes.
Degenerative Joint Disease: Osteoarthritis of the sacroiliac joint can cause pain and stiffness, especially in older adults.
Piriformis Syndrome: The piriformis muscle runs from the base of the spine to the thighbone, and when it compresses the sciatic nerve, it can cause pain in the sacroiliac area.

Septic Arthritis: Although rare, septic arthritis of the sacroiliac joint is a medical emergency that requires prompt treatment to prevent serious complications.
Osteomyelitis: Infection of the bone can cause sacroiliac pain and is a serious condition that needs to be ruled out.
Cauda Equina Syndrome: A rare but serious condition where the nerves in the spinal canal are compressed, causing pain, numbness, and weakness in the legs.

Sacroiliac Joint Tumor: Tumors, either benign or malignant, can occur in the sacroiliac joint and cause pain.
Inflammatory Bowel Disease: Conditions like Crohn's disease and ulcerative colitis can cause sacroiliitis and sacroiliac pain.
Hypermobility Syndrome: A condition characterized by excessive joint mobility, which can lead to sacroiliac joint instability and pain.