Causes of Sacroiliac joint pain can be split into four categories:
* Traumatic
* Biomechanical
* Hormonal
* Inflammatory joint disease


Traumatic
Traumatic injuries to the SIJ are caused when there is a sudden impact which 'jolts' the joint. A common example is landing on the buttocks. This kind of injury usually causes damage to the ligaments which support the joint.

Biomechanical
Pain due to biomechanical injuries will usually come on over a period of time and often with increased activity or a change in occupation/sport etc. The most common biomechanical problems include:
* Leg length discrepancy
* Overpronation
* 'Twisted pelvis'
* Muscle imbalances


Hormonal
Hormonal changes, most notably during pregnancy can cause sacroiliac pain. In preparation for giving birth, the ligaments of the pelvis especially increase in laxity. Combining this with an increase in weight putting extra strain on the spine, may lead to mechanical changes which can result in pain.

Inflammatory joint disease
Spondyloarthropathies are inflammatory conditions which affect the spine. These include Ankylosing Spondylitis which is the most common inflammatory condition to cause SI joint pain.