Oikawa’s fall from grace during the Karasuno v. Seijoh match
We all saw how after setting to Iwa ALL THE WAY ACROSS THE COURT WHILE MOVING and still managed to get it perfectly to him, Oikawa crashed into a bunch of tables and slipped on the carpet while trying to get back into play.
To me, this was clear foreshadowing (although it was pretty evident at that point) of how Seijoh was losing the match. He was desperate, doing anything he could to try and win no matter what. But even against his best efforts, how hard he worked, his work was all for nothing. He tried to get up but something he couldn’t control kept him down, much like the difference between his skills and Kageyamas. Him trying hard to be the best player he can for his team but someone else being better. Something that Oikawa feared ever since middle school where they played together and he caught a glimpse of, for the first time, the fact that he can’t be the best forever.
In short, this symbolized his fall from grace. He was acknowledged as “The Great King” by Hinata, meaning he was put on a pedestal and thought of as better than Hinata. But after Hinata defeated this immense and seemingly impossible to crush opponent, he can’t think of Oikawa as an absolute and indestructible power anymore. Kageyama can see that Oikawa isn’t as powerful as he idolized him to be, and Oikawa is finally being surpassed by Kageyama. Oikawa knew all this, and saw it in finality as the gap in points between the two teams widened.
And to make all this more painful for him, pointed out in this ask on @hailsousuke‘s blog, Oikawa fell while trying to get back up onto his BAD KNEE. He was in physical and emotional pain, fighting against the realization that he is being surpassed and the pain brought by his knee must have been neARLY UNBEARABLE. But he kept playing knowing that if he gave up, his team would no longer have a strong six players, despite the obvious fact that Karasuno’s six were stronger, proven by their points. He knew that everyone else was fighting till the end, that if he gave up he would never be able to play volleyball with this team again. As their captain he had a responsibility to his teammates to hold face and push himself to his limits, even if it meant fighting against a barrier of mind and bodily pain.