It doesn't. [He shakes his head faintly.] The Keyblade reacts to the heart of its wielder, so unless something happened that disturbed your heart, it should have appeared to you. [But if Riku had it originally, could it be that it was Riku's Keyblade, and that had somehow triggered Kairi's own Keyblade usage?

He sighs to himself.] Sorry. Even I don't know all of the mysteries of the Keyblade.