Because people keep wondering about it, the Wii U/NSMBU bump is indeed a result of Mario Kart 8 hype.
For the last two weeks, Nintendo has started touring MK8 around Japan and demoing it in electronics stores, such as Yodobashi Camera. It is getting a lot of attention, and as a result, the Wii U aisles definitely aren't as as barren as they were this time a month ago. This may be anecdotal, but I have also seen more people purchasing Wii U systems than usual, so I was wondering if there would be a slight boost ahead of MK8's release.
Expect a much larger hardware boost next week.