My oldest Leopard gecko, Neo, recently had a freak tail-dropping as well. I just looked in his humid hide one morning and part of his tail was just laying there. I took him to the vet and we still don't know why his tail fell off. Just put some neosporin on the stub (make sure its regular neo. not the kind w/pain reliever!) and watch out for infection. If you notice anything wrong w/him, consult a vet.