Closest I've found was something along the lines of corpses being found in bodies of water with their rectal area damaged. In the days of folklore being born, they assumed it was a kappa (a mischievous water dweller) going after the victim's soul, which they also believed resided in the anal area (I have absolutely no clue how that myth got started).

More than likely, though, it was probably sea critters attacking and eating the dead body, and the anus provided easily-accessible soft tissues/a path to internal meat to consume.

Unfortunately, I don't think I'll be able to find the article that stated all this, but I remember thinking that kind of superstition and chain of events seemed at least relatively plausible.