Nintendo didn't exactly explain why the service was down, but given that the Switch was one of the most purchased items during Black Friday, it's not surprising that some people have theorized that Switches given to people as
gifts were all activated on Christmas day and then when a
whole bunch of people decided to go online to retrieve the digital game codes that came
bundled with the system, or to activate the eShop cards, it brought the Nintendo Network down to a screeching halt.