Iga's arch rival Aryna Sabalenka had made it to the semis. She did reached the US open finals last year too but was defeated by American Coco Gauff in the finals. This time she's playing against another American Emma Navaro in the semis and possibly Jessica Pegula in the finals. Personally, I'd love to see Aryna Sabalenka win.
I could be wrong but I feel like this isn't a rare event when a great player goes undefeated for a long time and then loses a game that really nobody expected that player to lose.
You could observe the same at the US Open for Djokovic and Alcaraz. Nobody would have thought that these two guys have to leave the tournament that early. Now Pegula is still the no. 6 in the world, so it is not that big of a shame. But of course Swiatek was the huge favorite in that game. But as soon as these underdogs fight back, even the no. 1s can get nervous and this is what happened. I found it surprising that it was in two plain sets. I think that's even more surprising than the defeat itself. If it was a 3-set game where it was close until the very end, well ok. But this way was quite strange given the great shape that Swiatek was in.