Your muscles are made up of many fibers. When you use a muscle, not all the fibers contract at the same time. Some rest while the others work, and then they trade places. When you really challenge your muscles, the changeovers can get a little ragged.
Beginning yogis often shake quite a lot. As your muscles get stronger from regular practice, the fibers learn to trade off between firing and resting with smoother coordination. Your quivering will probably subside (though there will always be teachers who turn you into yoga jelly, no matter how strong you get).
To calm the body, try to hug the quivering (contracting) muscle against its underlying bone and press the bone into the muscle being stretched. Quivering is not necessarily bad, but it may be a sign that the body is overworked.