My guess is that it was children who began this folkloric game of telephone, because it's almost always the children. They hear a story, it begins to morph, they grow up, they repeat the story to their children, perhaps with a little spice added, and on it goes. Because children, then as now, seem to like to make any urban legend as gruesome as possible and one-up their friends, eventually we end up with a Christmas-themed scarecrow that eats kids.
Much like Santa Claus, Perchta was believed to visit houses between Christmas and Epiphany in search of who had been good or bad. If you had been a good child--worked hard and behaved yourself--you might find a silver coin in your shoe. If you hadn't, well...