[The Graceful Assassin]

Actually that scene is very vague on how and why it happened. It's left open to debate, but you should recall that Roxas was beating Sora and had basically won, but Sora used that cheap move where the keyblade returned to him and then he struck Roxas. Plus, it's not like Roxas died, so he didn't actually beat him. It was more of a test for Sora