I think he got veneers for his teeth and they might have thrown off his mouth coordination just a bit. We never really think about coordination until we bite our own tongue or cheek and then you keep doing it because something no longer fits. Well, get thicker teeth and this can throw 70 years of mouth coordination off a bit. That's my theory and I think he had them trimmed after he had trouble speaking in early campaigning. Biden was one of the early adopters of hair plugs, so he's never been shy about looking the part.