Having read it, I can believe Roberts changed his vote because -- and I do not say this lightly -- the four dissenting conservatives went totally batshit crazy on two issues: the Medicaid expansion and severability clause analysis. I can actually see how Kennedy's opinion would have driven Roberts to a much more moderate position on both these key elements.
I think the dissenters are also contrary to precedent on their analysis of the tax issue, but they aren't batshit crazy on that the way they are on the other two.
Since most people will never actually read the opinion, and don't care much about anything but the outcomes, this will be lost in all the noise and analysis. For lawyers who care about lawyering -- which includes Roberts, based on everything anyone ever said about him before and after his confirmation -- it's not just what you do. How you do it is bloody important, especially at the Supreme Court level. There is also a huge difference between making a hard-bank turn on matters where scholarly debate has raged for years and pulling stuff out of your rear-end.
For example, if you look at cases where the Supreme Court has reversed course in the last few years, e.g. Citizens United (which reversed McConnell) or the Second Amendment cases (finding gun ownership a personal, rather than a collective, right), you will find lots of scholarly literature arguing the points. The results did not just drop out of a clear blue sky from nowhere. Everyone could guess where the trends in law were going precisely because there were years of law review articles, lesser decisions, and dicta in various cases laying the groundwork before the Supreme Court announced a significant change in direction.
None of that is true for the conservative dissenters in the Medicaid expansion and the severability analysis. to the contrary, it is telling that the Kennedy dissent can go for pages at a time without a single citation. Worse, the dissent is nakedly political in its analysis. It reads like a Tea Party manifesto, in a manner that would have utterly destroyed the credibility of the institution in the way that Lochner and its progeny did a century ago.
To be clear, I don't refer to the Court's (or the Kennedy dissent) analysis of the Commerce Clause or the necessary and proper clause. While the words are sharp on all sides, those presented close issues of law (IMO). The majority is, I believe, ultimately correct that the Court has never sanctioned a requirement to affirmatively engage in commerce under the Commerce Clause, and that while a failure to get insurance is economically the same thing as an affirmative decision to go uninsured that is not dispositive under the Commerce Clause and a theory of enumerated powers. Same with necessary and proper.
Where things begin to get funny is when the Roberts opinion looks at the Tax argument. Here the dissent starts to veer into clearly outcome-determinative territory. Certainly what Congress chooses to call something is informative, but has never been determinative. As Roberts points out, there is a substantial jurisprudence holding exactly the opposite. The Dissent argues that it is impossible to consider the Tax argument because Congress clearly believed it was acting under the Commerce power. But this has never been the test for Constitutionality. Congress is under no obligation to specify under what power it acts, and the Court is required to consider any argument that may save the constitutionality of the statute. The Dissent essentially argues that even if Congress could have gone back and relabeled every place where it says "penalty" to say "tax," the Court should find the statute unconstitutional and require Congress to do so. But this is the opposite of the well established principle relied on by the Roberts opinion that the objective of the Court should be to find the statute Constitutional if there is any construction that will do so.
(Ginsburg has a valid argument that the Court should therefore have avoided a finding under the Commerce Clause, but I think Roberts has a valid counter argument that the Court should not address an alternate theory when the government advances a clear argument in chief.)
We now get to the Medicaid argument, and this is where the Dissenters totally flip out. They express outrage that states could be compelled by the fact that other states accept the money -- thus creating a situation where states that refuse to take the money subsidize states that do. LEt us set aside that this was exactly what happened with Medicaid from its inception until Arizona, the last hold out, accepted Medicaid in 1981. This principle has absolutely no basis in any case law -- as evidenced by the Dissents' failure to cite any cases. It is also absurd on its face. All taxes result in an uneven distribution of subsidies among the states. This is the natural consequence of the concentration of wealth in a handful of states and the concentration of poverty in other states. It is also a direct contradiction of every precedent the court has ever had to resort to an argument that presenting voters in states with an obvious choice is "coercive." To the contrary, the Court has always been careful to observe that such choices lie in the hands of the electorate and that Governors or state legislatures that wish to reject federal funds make the case to their citizens.
The severability argument follows the same departure from established precedent and descends into pure politics. In finding that Congress would not have passed various provisions in the absence of the mandate or the Medicaid expansion, the Dissent resorts to armchair psychology of a kind never seen in a severability case. "Well, Congress would not have passed a reduction in Medicare if it weren't for the Medicaid expansion because that is hard to do politically." WTF? All complex statutes represent political tradeoffs. The entire purpose of the severability clause is to make it possible for provisions to survive when other provisions are found unconstitutional. This is why the entire Telecommunications Act of 1996 was not made null and void when the Communications Decency Act was found to be unconstitutional. Do we know that members of Congress would have voted for the 1996 Act absent this provision? Of course not. Heck, do we know that Congress would have approved the "safe harbor" provision of Section 230 in the absence of the content provisions of the CDA? Of course not. And that is why severability clauses don't work that way.
To conclude, I can totally see why Roberts would not want to go along with the Dissenters, and this gives the theory that he changed his vote some credibility for me. Mind you, this is actually how judicial drafting works. I have clerked, albeit for a lesser court, and saw on several occasions where a back and forth in the written opinion prompted one judge on the panel to change his or her mind and thus reversed the verdict. It is also possible that the Dissent beefed up their language when they moved into dissent. Whatever, the actual dissent is pretty shocking.