Because he screwed up Rukawa's previous attack against Sawakita, Sakuragi makes up for it by stopping Sawakita's subsequent attack. 

It may seem Sakuragi's just doing it because he made a total ass of himself previously, but he just so happens to mention that he did do it because he felt he owed Rukawa something. *cough cough* 

I'm sorry, it's just that, you know, I'd thought that Sakuragi would relish any chance he'd get to totally mess up Rukawa's playing (flashback to the numerous times he did exactly that in just about every game they've ever had). It's not even a question of why, because it's obvious that Sakuragi's starting to look at Rukawa as more of a teammate/ally rather than an enemy.