My friend Kate and I were talking about our dodgy sacroiliac joints the other day. The topic came up because hers is hurting right now, following a popping noise in an early morning yoga class. Popping noises, when related to SI Joints, are alarming things. Popping noises, when related to ANY joints, are alarming things. Now, for the scientific lowdown on SI Joint dysfunction, please read this excellent wikipedia article.
I will add that in yogis, SIJ dysfunction is almost always a result of acquired hyper mobility in those joints. This hyper mobility generally develops over time, by repeated insult to the ligaments that hold the joint stable.
Extreme adduction (crossing the legs over your midline) and abdcution (taking your legs out to the sides), especially with bent knees, as in some of the poses below, is fine for many people. But not all. As a result of how our bones are shaped, and the loads and forces we subject them to (think tightening your psoas muscles and other external hip rotators with years and years of sitting), not all of us can move our legs in such a big range of motion in our hip sockets, and so, when we are trying to get into poses that we don't have the mobility for, we inevitably recruit flexibility from other joints - knees and SIJ's usually.
Over time, the ligaments that stabilise these joints get stretched and damaged, and the joints get destabilised. Not pretty. Very painful. Not necessary, but once it's happened, you live with the consequences for, probably, ever. Ligaments are generally accepted as being avascular - that is, they don't have their own blood supply, and since their job is to staibilise your joints, they are not very elastic either. Once they are overstretched, they don't go back to their original length.
(Side note: I found prolotherapy (a simple injection into the ligaments which causes them to form scar tissue & therefore tighten) useful, but the ligaments in my pelvis are still less stable than those of other people.)
My early yoga teachers didn't know this stuff, or if they did, they ignored it. I didn't question their teaching: I was too green back then to know that I don't bend right for yoga.
I got stood on in Baddha Konasana to get my knees to the ground (they still don't go to the ground, but now I have an unstable pelvis. Thanks, teacher, for standing on my knees).