Listed below are the major reasons for pain around the sacroiliac joint:Muscle tightnessPregnancy: the pelvis widens to prepare for birth, stretching the ligaments -- strong, flexible tissue that connects bone to boneDifferent types of arthritis that causes SIJ painDifference in leg lengthsWearing away of the cartilage (cushion) between the bonesTrauma from impact, such as landing hard on buttocks