Are you trying to pet it when it happens? If so, how are you doing it? If ya lunge in, it will be frightened and the natural reaction is to bite in defence.
Always slowly approach your rabbit from the front and slowly try and scratch its nose to gain its confidence first. Just imagine if you were the rabbit and saw this large arm/hand coming towards you. Whatever you do try and pretend the roles are reversed and imagine if you were the rabbit how you might feel.
Try not to be afraid when approaching your rabbit as it will sense your fear. Talk to it calmly and offer it some apple or something it likes. Do this for a while till it gaines your trust. Then start to gently pet it being very calm, when it has got used to that start to pick it up for short amounts of time until you build up a bond where you both feel comfortable with each other. It will take time and patience.