This is monstrous, but actually a bit expected IMO.
SwSh had years of tremendously strong word-of-mouth, which we still see in its colossal legs. It's the leggiest Pokemon game in 25 years, and the best-selling. Repeat sales due to highly positive reception. When that happens, you can sometimes expect an absolutely explosive opening for the sequel. We see it a lot in movies. For example, Curse of the Black Pearl -> Dead Man's Chest or Batman Begins -> The Dark Knight.