The way to reproduce this that we found was to recline on your back at about 45 degree angle (gaming chair style) and simply move the head a bit. What you'll observe is the hip swinging wildly forward and back, as if moving between the tracked position and the animated position of crouch animation.
While in head lock mode hip drift is inevitable in some poses, I'd expect the hip to at least stay constrained to the line going through head and hip trackers (i.e. it drifts down, away from head), instead of just going to completely random positions unrelated to hip tracker position.