Random, but the character dynamics in this manga are always so good.
Like, it's such a small detail, but it's pretty clear that after they wrote on Okazaki's face (or maybe Anjou was the one who did it), Anjou saw the opportunity, grabbed the marker, and started messing with Seto's face. It's a minor thing, but it really gives you that feeling that things keep happening outside the manga's immediate focus in a very nonchalant way.