In my experience, this is a sign that you are - possibly unawares - doing something that slightly annoys the cat and a light bite is his/her way of telling you to stop. My beast, for example, absolutely hates it if I use the computer when he's on my lap. On other occasions, I may be stroking too close to his stomach for his comfort - he's very ticklish, as it were! That, too, earns a nip.