Anatomically Hanumanasana is very interesting in that it is an extension (backbend) with an equal and completely opposite flexion (forward bend). In both backbends and forward bends part of the movement comes in the legs and the back. In-between this we have the pelvis and spine which must stay in neutral which takes a lot of the stretch into the legs. I would like to say at this point though that if we practice flexions correctly the lumbar spine will retain its neutral curves, and we will be better prepared for hanumanasana.
Practicing forward bends with a rounded lower back will accentuate with a posture like Hanumanasana, because the spine and pelvis needs to stay in neutral. Also in our eagerness to master this fairly extreme stretch, we tend to put most of our effort into the leading leg, forgetting the trailing leg. The trailing leg, either because we forget to engage with it or because we put the effort through the front will laterally rotate (falls away from the mid line) As we push into our front leg, with an unstable pelvis, this causes the leg to also roll away from the mid-line. This is evidenced when the pelvis is either not flat to the floor, or the thigh of the back leg is not grounded.
This action tends to put pressure on the pelvis causing it to twist. This part of the spine is intended only to flex and extend (forward and backward bending) The twisting part of the spine is much further up in the thoracic spine. The sacroiliac joint is below the lumbar spine and above the tail-bone, it is kept stable by strong ligaments attaching it to the pelvis, if we persist in pushing the sacroiliac joint into a twisting action this will weaken the ligaments leading to sacroiliac joint dysfunction. Once we stretch those ligaments there is no coming back, so beware.
We also have to consider gravity when practising Hanumanasana, in most other postures we need to engage with muscles to pull limbs into position. This is not the case with Hanumanasana, if we push too hard, or relax to much the legs will automatically roll outwards. It takes quite a bit of effort to keep the legs in neutral in splits, in fact there has to be a fair amount of inward rotation of the legs just to keep them stabilised.
This asks the question what is the correct way to enter and progress into splits safely. We must take action to reach past our bodies acceptable range of motion, too much action will lead to the potential problems already mentioned. Too little action and we hang listlessly never daring to take the body beyond what it thinks is normal. It means we must fully support the posture, letting gravity take control will not work. Progress will take place when we are ready to accept letting go of our patterns of behaviour, this does not mean giving up responsibility, quite the opposite we need to accept full responsibly. Once we do the body will let go of its resistance and we will express Hanumanasana with grace and poise.