Molly, my sister's cat, does this all the time. She will usually smack her tail down hard right before she bites to at least give us some warning. I once saw an episode of the "Pet Psychic" where they addressed this issue. Sonya Fitzpatrick said that it was static electricity building up in their fur. They sometimes get little electric shocks that we aren't aware of. When it hurts too much, they bite.