A piggybacking trailer is a truck trailer hauled on a railroad flatcar; a piggyback plant is a perennial herb—a foliage plant—native to western North America whose young plants are borne at the junction of leaf blade and petiole. A piggybacked space capsule is carried into space as an extra load by a rocket or other spacecraft. And piggybacking onto someone’s wireless or cable signal involves a virtual ride on another’s power. (来源：http://www.EnglishCN.com)

Not one of these senses is associated with the barnyard pig; but all have piggybacked—that is, have functioned or carried as if on the back of another—on the sense of piggyback that originally meant "up on the back and shoulders" and that is commonly used in the sense ride piggyback. The porcine pig is not responsible for the transfer (or appearance) of the pig in piggyback; piggyback is, in fact, an alteration of the older "a pick pack; a pickback".

So what’s the story behind pickback or pickpack? Except that it too means means "on the back", we don’t really know much about it, besides the fact that it has been largely replaced by the more familiar piggyback.