Actually, it's none of those reasons. Originally, hogs were dropped in boiling water, scalding them, then the hair scraped off the skin. This boil also solidified the layers of fat under the skin to the fascia on the outer membrane of the meat, making it firmer. Scalding lost its favor in place of skinning; once a fresh-killed hog is skinned, the underlying fat layer is almost like jello. Dropping it in boiling water again 'solidifies' the fat layers into more cuttable form and boils off any stray hairs that can be bacteria-laden from the exterior of the hog, esp. in the hindquarters with guts and feces easily contaminating the surfaces and stray hairs. It's a good thing to do.