Nail biting is a common symptom.  It is simply the remnant of a childhood behaviour that gave comfort (eg thumb sucking) and as such, nail biting typically increases during periods of stress and uneasiness.

The behaviour is unconscious and many people say they only realise they are biting their nails some seconds into the action.  Hypnotherapy/therapy is directed towards bringing it in to conscious awareness in turn allowing you to gain control again of this symptom.