Well there was always the question of S2 sales losing some steam after couple months on the market, given the jacked up price of the system (and everything else in the S2 ecosystem) and the only big first party game for months after launch day being DK, which, while apparently being a great game, is a game trying to revitalize an old IP rather than being a consistent huge system seller like 3D Mario would be which is the game everyone expected to hit during the launch period. Japan especially I would expect DK to be less of a system seller.
The launch sales for S2 were always going to be huge, but the big question has always been (ever since that early April S2 reveal Direct ended with no Mario and crazy pricing), after launch hype is over, and there's no system seller on the horizon other than maybe a cross-gen Pokemon that doesn't particularly look amazing, how many people are going to be eager to pay $450-$500 for the system.
Aug/Sept is when I figured sales might slow down to a long term baseline, a baseline very different than what the launch hype would suggest, if the crazy high prices end up being a problem for the market. Can't be sure that is what is happening - maybe just a down week, maybe it's just Japan, maybe it's temporary lack of supply, maybe it'll just be somewhat slow until Pokemon, or maybe we're gonna see the outrage at Nintendo entirely abandoning affordable gaming was the real deal and day 1 buyers simply tried to downplay it during launch month hype.
Time will tell. But definitely right now not a good look that launch numbers were overstated by a quarter million, weekly sales are well under 100k now and already trending down to the 40k-60k long term Switch baseline, and DK is looking like a relative miss in Japan just re-emphasizing that they should have launched with the guaranteed system seller 3D Mario. If they had Splatoon or Animal Crossing or a next gen Pokemon releasing in the coming weeks it might be a different tune, but with only a cross gen Pokemon over 2 months away and at least so far no other system sellers announced for the year, things might start getting a bit lackluster now as plenty of people were pointing out in April, now that the launch is in the rear view mirror and there's not much in the way of system sellers in sight.