No idea about the piggys yawning sorry but reading your thread title made me yawn like mad
mine yawn when they're relaxed. Sometimes I see them do it a lot, sometimes I don't see them at all. I think it's probably just a case that you've noticed it.
Maybe you spend too much time watching your pigs
my buds does it everytime I stroke his back and lays flat out, he loves it, its cute to watch them yawn