The recent downtime for Nintendo Network and the Nintendo eShop was caused by "a one-time surge in demand" during the holidays, according to a post on the company's Facebook page.
The network began crashing on Christmas Day. During that time, Nintendo Network experienced "an influx" of users both creating new accounts and attempting to access the eShop. Users transferring existing Nintendo 3DS account information to new Network accounts compounded the problem.
Nintendo took the eShop offline yesterday at 4 p.m. ET in order to perform maintenance. More system maintenance might be necessary in the near future.
"We truly apologize for the inconsistencies in our online services," the post reads. "We are working hard around the clock to fully restore these functions, and will continue to keep you updated on our progress."