The main concerns are primarily:
- Switch sell through is lower than expected in H1 2018
- Nintendo Labo launch was muted and isn't selling Switch hardware
- Nintendo Switch Online service won't launch with a killer app
- Strong competition from other Console, PC, Mobile games
Of course whilst that is some what of a concern, there is no doubt that Smash Ultimate will be the biggest Smash game ever. Pokemon tying in with Pokemon Go / going back to Yellow will really help with sales. Also Nintendo is doing well in other segments such as mobile etc...
MOD EDIT (July 4): Down another 5.27%
https://gonintendo.com/stories/3128...o-drop-nikkei-says-slow-switch-sales-a?page=1
The Nikkei thinks it's because of a lack of new information on the game pipeline and sluggish sales of the Switch, causing foreign shareholders to get rid of the stock.