Okay, I'm seriously smacking my forehead right now. From Equestrian Life - Understanding the Sacroiliac Joint: PELVIC ROTATION – Sacroiliac injuries are characterised by visual asymmetry or rotation of the pelvis. ... This rotation is sustained or held in place by the powerful pull of the large back and pelvic muscles. It occurs in an attempt to alleviate pain following the initial injury.
Now I feel awful for brushing off SI injury last year just because he wasn't sore on that part of his rump.