I think I see what you mean. The idea is that there is in MMA a fairly constant level of threat of a low-level attack to the legs. My guess is that the defence to a low level charge to the legs would be to pull the legs back looking to ensure that the opponent doesn't grab them and allowing the body to 'fall' on top of the incoming opponent? This would I guess explain the crouch-style stance and the bodyweight being allowed to go slightly forward?
It's interesting. Seems that a key moment is when a fight switches from stand-up to grappling-type stuff.