I've had two tailbiters in the past, it can be triggered by anything, even just boredom. My halfmoon bit his one day then let it regrow then bit it off again and thats how he continued, my halfmoon double tail bit his off when he was posted to me and just kept biting after that. There's no garantee they will stop doing it, all you can do is keep the water clean and make sure finrot doesn't set in. Hope they both perk up soon!