I'm sure you've heard about the GOP tax bill, which has been dominating headlines for the past few weeks. The decision to lower corporate rates will have a huge impact on certain industries. The result is a rotation from tech stocks to other sectors like financials. The plunge in Nintendo's stock price is not an isolated event. If you take a look at the stock prices of other major game companies like Activsion Blizzard or Take Two, you'll notice a similar drop within the same timespan.
It wouldn't affect the Japanese stock exchange lol. Zero effect. It was absolutely the Animal Crossing news. Which is why Nintendo is already recovering since new was released that it would start publishing games in China. That and Mario Run is the top selling game on iOS in 2017.