I'm of the school of thought that "it just happens," "freak accidents," and "random occurances."

I just had and ASF born with no tail (sadly, I just fed him off tonight).

In the rats (not asf) in the first year, I had a trio three headed baby (well, the body was one body and a lot of appendanges and I think the mother put it out of it's misery because the head was chewed off.)

I believe I still have the pics, but I don't want to derail the OP's topic.