This story may give a clue as to why we cover our mouths when yawning:
“Once, a man was standing outside his inn, ‘when a black dog came up and stood in front of me and yawned, which made me yawn too, quite against my will, and immediately the dog disappeared from sight and I was seized with fever and my face was turned round backwards.'”*
So, according to St. Theodore of Sykeon (600s CE), an unprotected yawning mouth gave access to sickness and demons who could make your face turn all the way around like in the Exorcist.
Maybe, if he’d thought of it, he might’ve realized that the dog was probably just Harry Potter’s Sirius Black messing with him–I mean, he did escape Azkaban and turn into a black dog and… Oh, wait. Wrong dog. Wrong story.
Never mind. But now tell the truth. Did’ja yawn when you looked at that picture? Did’ja cover your mouth?
*Source: Popular Culture in Ancient Rome by Jerry Toner