It is certainly possible that the golgi tendon organs (GTO) fire while in any yoga pose, yin or yang. These little sensory cells are found in the area where the muscles become the tendons (called the musculotendinous junction). Their purpose is to notice when there is too much stress on this area, and if so, they send a reflex signal through the spinal cord telling that muscle to relax, and thus reduce the stress. Many therapists and even some yoga teachers deliberately try to trigger this relaxation response to allow the muscles to relax, release and lengthen (it is part of a practice called PNF, or proprioceptive neuromuscular facilitation). While you marinate in a yin yoga pose, there is often stresses in the muscle and tendon, and this could trigger the GTO to fire and further relax the muscles. Part of that release you feel when your edge releases and you are invited to go deeper in the pose may be due to the GTO activation.
Cheers, Bernie