its probably because of the ludicrous 'analyst' predictions of it doing like as well as Pokemon GO> which was pretty silly. Also a massive amount of devices at this point are on ANDROID, probably overall worldwide way more than IOS, so some of the predictions seemed a bit far off considering essentially half of the market can't access the game right now
regardless the stocks always seem to be super sensitive with Nintendo. too many businessmen who don't pay attention to games and are trying to make a quick buck. Nintendo releases a mobile game that does fairly well upon release, when they've never had any mobile game profit coming in before, which should be looked at as a positive but instead people freak out because they expected Pokemon GO success over and over (which is silly).
Mario Run is doing fairly well for what it is, the NES Classic is doing well (despite the major undershipments). the stocks are just bizarre sometimes