There is a massive system seller already coming in march.
Also Switch is down partly because of the Smash Ultimate spillover.
Nothing of importance released last December. Brain Age did not get a release in the US.
But Animal Crossing will turn things around. Watch Swich be up the next 6 months easily.
Regardless of the reasons, no Nintendo system that has, after experiencing several consecutive quarters of growth followed by several consecutive quarters of YoY losses, ever experienced another period of growth. Once the growth gave way to losses, the system was post-peak (this isn't always the case for PlayStation & Xbox, for reasons too complex for me to want to get into at the moment).
The Switch's sales growth was already clearly slowing down, and it's already down two consecutive months so far. If it continues to have YoY drops well into the year, that will effectively signal the Switch's transition to post-peak in the U.S.
And I don't expect Animal Crossing to do wonders for Switch sales, either. It may provide a decent boost for March, but that's it. If you look at New Leaf's performance in the U.S., it sold only 1.36M copies in its first 12 weeks, and its effects on 3DS hardware sales helped it for maybe two months at best and even then only relatively modestly, with Pokemon X & Y being a far better system-seller: