According to the changelog on the Minecraft wiki, only Nitwits and unemployed villagers are supposed to shake their heads when attempting to trade with them. However, if you have attempted to trade with one before they gain a profession, they still nod their head when you right click on them afterwards. (Unintended behavior) This doesn't happen if you don't right click on an unemployed villager before they gain a profession, they don't nod their heads. (Intended behavior)