Sometimes shaking is a natural symptom of trying to release stress. All animals do it. People do it to when they want to "shake things off" -- its quite natural when people feel trauma or intense emotion.

There are some really effective therapies -- Trauma Release Exercises and Bioenergetics that are solely focused on shaking and how that relieves one of stress. Check em out on Youtube.

That is if though its coming from just overwhelming emotions and is not health-related.