According to Nintendo’s Facebook page, the recent Nintendo Network and the Nintendo eShop outage was caused by “a one-time surge in demand” during the holidays.
It all started on Christmas Day when the Nintendo Network started experiencing an increase of users creating new accounts and attempting to gain access to the eShop. Users transferring existing Nintendo 3DS account information to new Network accounts further added to the problem.
As a response, Nintendo temporarily closed the eShop’s virtual doors in both America and Europe yesterday at 4pm ET to perform much needed maintenance.
Nintendo updated users by releasing this statement,
We truly apologize for the inconsistencies in our online services. We are working hard around the clock to fully restore these functions, and will continue to keep you updated on our progress.
Further downtime due to system maintenance might be necessary in the near future.