Please use this forum to ask any questions you may have about yoga in general or Yin Yoga in particular, or to discuss anything you have discovered that may be of general interest. Note, spam will be removed and the user deleted, and this includes putting website in your posting that are purely commercial.
1) I don't know what is causing your reaction. It could be soooooo many things. You want some speculations? Muscles, when tired, sometimes spasm (tremble) or cramp. If you have a deficiency of electrolytes in your system, it can happen. Not enough calcium in your diet could cause it. If you are tired, exhausted, stressed, it could happen. If you have a serious neurological disease, trembling could result. You could simply be a natural trembler and you got it from your father (or mother). So, it may be the nervous system, but it could be many other things too.
2) Is it good? What does your intuition tell you? Does it feel good to you? Is your body happy? For me, it is not a normal state of being, so I would personally not want to go to a point of trembling, but some exercise instructors try to push their clients' muscles to a state of ultimate fatigue: for them, they think it is good. But, they're your hamstrings…what do you think?
You could have a look at the muscle spindle stretch receptors http://www.bandhayoga.com/keys_fire.html . It's part of the nervous system that causes the muscles to contract when they are stretched. Try the instructions in the link and see if it works!
