Why are Pokemon in SV surprised when you throw a ball at them?

The Pokémon can see you throwing a ball (with a Party Pokemon) at them, then are surprised (? ; they go "!" anyway), then sometimes go "?" or "!" again quickly after. What would surprise them if they already see the ball being thrown at them? Being hit by it? The balls don't seem to hurt them, so is it a quick adrenaline rush or excitement (like being hit with a foam ball)?