Check the NPD prediction thread and you can see that XBO got boosted by the 80$ pricecut for the S and 50$ for the X. It's not the Switch sales that dropped, but the Xbox sales that jumped up with the promotion and good E3 show.
Next week will be similar (with slightly rising PS4 and Switch sales from their Shows), but the week after that will be interesting. In the NPD predicition thread (which has been spot on with Switch and PS4 sales until now), PS4 daily sales started to drop below Switch daily sales since the 50$ promotion of the PRO ended. Granted, the advantage is small and won't be enough to keep Sony from winning another NPD, but it shows that the dry season for the Switch is over and sales are recovering from their spring low. A similar thing can be seen in Japan too, where the Switch sales went up every week for 6 weeks straight, going from 30k to 48k
Didn't notice the XB1 price drop was temporary. It may not surpass the Switch after all in NA. I don't expect the "dry season" to really end for Switch sales though, not until the holidays. Mario Tennis isn't really going to move hardware like Smash Bros or Pokemon Let's Go.
The thing is, it's not just Mario Tennis Aces. With it comes a lot of third party titles (not just Indies like most of the games in the months prior), third party exclusives (Octopath Traveler, Monster Hunter Generation Ultimate) and some awaited DLC for Splatoon 2 and XC2. While there's not much from Nintendo themselves, having all these games releasing in that timeframe more than makes up for that.
Oh, in case you're wondering what 3rd party titles I meant: Wolfenstein II, Crash Bandicoot, Shining Resonance, Sonic Mania Plus, Megaman X Legacy collection 1+2, Overcooked 2, Okami, Dragonball Fighter Z, Dark Souls, Valkiria Chronicles 4, Disgaea 1, Steins;Gate Elite, Super Neptunia RPG; all these titles are supposed to come out until the end of September (2 only have a Q3 release date) and is a much bigger lineup than what Switch had until then in terms of third party support.