Most people presenting with sacroiliitis usually complain about pain in their buttocks or the lower lumbar spine. What usually they will say is the pain kind of goes across the lumbar spine and usually gets worse when they are standing for a long period, walking, and very commonly when they are transitioning from sitting to standing. Now this pain usually is what we call "axial" - in other words it doesn't really spread down the legs or up the spine, although sometimes it can fool us and the pain from the sacroiliac joint can resemble pain that radiates down the leg, although it is unusual for it to radiate below the knee.
There are a variety of causes for sacroiliitis. Sacroiliitis can come from intrinsic problems of the sacrum and the ilium - in other words, if there is a sacroiliac dysfunction or the mobility of the sacrum itself is altered, that can cause inflammation in itself. Of course, the inflammation can be caused by altered mechanics of the joints surrounding the sacroiliac joint.
As physicians treating sacroiliitis, we always have to be aware of the fact that there are other systemic problems and other systemic inflammatory diseases, such as spondyloarthropathies like ankylosing spondylitis that can cause pain in the sacroiliac joint, inflammation and even though they are systemic manifestations of this disease, many times sacroiliitis is the first presentation of the disease. Sacroiliitis can also be caused by trauma. The impact can come from different directions and still really impact the sacroiliac joint. Trauma that is caused by missing a step or falling down and actually landing on your feet, but unevenly, can cause sheering forces through the sacroiliac joint and cause significant inflammation. Also, trauma from impact coming from the side compressing the sacroiliac joint or even trauma into the leg expanding it can cause sacroiliitis and inflammation after the impact.
Life events like pregnancy - during pregnancy, all the joints become loose and lax as part of the process. After the birth, what we're usually hoping is all of these joints will come back with their natural state and function like they should. At times they don't. So, one can present with problems of sacroiliitis during pregnancy, because of the laxity of the joint, but also after pregnancy and after giving birth that can persist as abnormal motion and can cause sacroiliitis.