I have a theory that those whose anuses turn inside out while shitting as an oldster partook in anal sex when younger. If you think of the engineering problem you'd face constructing a tubular rectum that can pass solids using pressure without itself being passed out of the body, you'd probably string a bunch of little tendons to hold it inside. If those things got stressed and torn by being abused, eventually the whole thing is just going to pop out.
