From article:
“If one party is clearly in the majority, then I think it makes the most sense for the state of Kansas for me to caucus with the majority,” he said earlier Wednesday.
My honest opinion is that he's saying that so he can appear to be the real independent, uncorrupted by Washington. When the rubber hits the road he's more in tune with the Democrats than the Republicans, and if he wanted to caucus with GOP I don't think they could trust him to always vote they way they need him to when they need him to. Think Susan Collins, only even less reliable.