It's probably the result of several hours of crying (like laughing can do) after a good hard bullying session.

"Hey Vaan! You little b!$%@! You sound like a girl when you talk! I also think you might be gay! How does Balthier feel about having a wimp like you in the party?"

You know, stuff like that gets the crying going and then BA-BLAM, you've got cry-baby abs.