Diarrhea, constipation, irritation from surgery (especially
descenting), and other things can cause a ferret to strain more when
defecating, which in turn can push a portion of the rectum out the
ferret's anus.  It's similar to hemorrhoids, but the particular tissue
that leads to hemorrhoids in humans doesn't exist in ferrets.

If only a small portion of tissue (1-3 mm) is protruding, a softened
diet and creams such as Preparation H can help.  If there are any
other symptoms (constipation, pain, diarrhea, redness or swelling), or
if more than 3 mm (about 1/8 inch) is showing, have a vet look at it.