Within the weight-bearing portion of the SIJ is the interosseous ligament, which contains blood vessels, connective and fatty tissue. This ligament is not a single continuous band. Rather, it is composed of many individual bands that go between the sacrum and ilium. When sprained, these interosseous ligament bands do not tear. Instead, they pull away from their insertion into either the sacrum or ilium; this allows the joint to separate slightly. Fluid from the surrounding tissues, as well as from blood vessels within the joint, infiltrates the space between the sacrum and ilium and creates swelling, inflammation, and pain. Here is the key point: the fluid within the space keeps the joint surfaces apart, preventing the re-integration of the ligament into the bone.
It is reasonable to assume that the internal pressure caused by the swelling pushes out on the joint surfaces, stretching the ligaments. Nerves within the ligaments respond by activating the ligamento-muscular reflex which, in turn, causes protective muscular contraction or inhibition. The internal pressure of the swelling, pushing out, is countered by the external muscular compression, pushing in, resulting in the body’s best attempt at stability.