Rumplessness is an inherited characteristic, resulting in the absence of some bones at the base of the spine and incomplete preening glands. It occurs naturally in about one in 1,000 chicks, but is an established characteristic of this breed. Aldrovandi mentions it the Rumpless Fowl in his treatise on chickens, http://tinyurl.com/6opssp, the first printed work devoted to the subject in 1600.
This is a historic breed with interesting connections to our modern world. It's one well worth saving.