User blog comment:JustSomeDude.../Why Zou?/@comment-1908942-20151216020225

My theory is that Momonosuke is actually the son (or grandson) of the Shogun of Wano, and there's some sort of conflict there that the Hundred Beasts Pirates are a part of. And to save the heir from danger, he was sent to Zou, a remote island that is constantly moving, to keep him safe. And Kin'emon, one of his bodyguards, is claming he's his father to avoid suspicion.

Now for Raizou. He is a ninja, and what ninjas are is spies and assassins. Of the trio guarding him, he is the most likely to know any secret orders the Shogun issued regarding Momonosuke, hence why he was specfically targeted.