How to Make a Pig Tail Straight
Adam Rifkin stashed this in Best Videos
Top Reddit comment explains why:
I think it has to do with reproduction. The pig shown is probably a female and when it senses something on her lower back, well she makes herself comfortable and braces for impact.
EDIT: Just to add that rubbing the same area on a female cat will make her raise her tail uncovering her reproductive system. Ehm.
EDIT2: Someone mentioned this is called Lordosis, please give him an upvote so that he can climb up the karma ladder. Link