Because Karasuno's plays become messy and the match's rhythm gets thrown off, they change setters. Sugawara goes onto the court in place of Kageyama and because he's always seen things from outside the court, he says he knows what's going on, and Karasuno starts their counterattack. Meanwhile, after seeing the match from the outside, there's a change in Kageyama as well.
Source: crunchyroll