Literally no one knows or cares why, let alone "exactly" hahaha!
"Playfully biting his wife's finger"
Yes, while attempting to become POTUS, in fact, at an event intended to gain support to achieve that goal. And yes, I mentioned I'd cropped (lol at "zoomed in") it as the original is quite a wide shot with many other things going on and imagery unrelated to his actions in the moment. Nothing has been changed though.
And please feel free to get as offended as you like that this moment happened and was captured in still so clearly, but also recognize that it did happen. I haven't made anything up here.
Our POTUS is old enough (by many years) to understand that a man must live with the legacy of his actions in the world. He chose to perform this action, and there is no misintention in someone posting the result online.