Mercen-X is right, but before Malificent had any influence over Riku, Riku went bad, kind of. Riku was desperate, he would do anything to get off of the Destiny Islands. Anything, including following the darkness in his heart. He wasn't afraid of it, he accepted it as a viable way of things. As he did that, the keyblade chose Sora instead of Riku. When he was desperate to find and rescue Kairi, he again followed the darkness in his heart, and it slowly began to twist and consume him (Episode III anyone? ), until Ansem finally convinced him to give into the darkness completely, allowing Ansem to take over his body. When he was in the Hallow Bastion, and he had found Kairi and he knew how to potentially save her, he for once saw the light, and that light allowed him to take control of the keyblade that would have been his in the first place. But when Sora showed the keyblade the pureness and strength of his heart, it overpowered Riku's heart, even if only for an instant, which is all he needed.

Kairi's heart with Sora? Call it the effects of the paopu (sp?) fruit (at least that's what the reference to that fruit is suppose to signify). It's also possible that the keyblade, or the power of the keyblade master, had something to do with it. Who knows. But fact of the matter, Kairi flew into Sora and then vanished, so what Mercen-X said.