A reader who clerked on an appellate court thinks very possibly:
[T]he jointly-written dissent was intended to be majority opinion. Any informed reader would reach that conclusion after the first three pages --- and then again when seeing the fact that any and all problems with the actual majority decision are addressed only at the end of it, following an oddly placed " * * *."
The following is speculation, but plausible, and would be an interesting parallel to the conservative legislative strategy. Any objective legal observer would tell you (and I'm trying to be one here) that the dissent's treatment of the severability issue is detached from 200 years of constitutional law. It's unsupported legally and it's a mess logically. It also includes a citation to a quote that Harry Reid gave to the New York Times in Janauary 2010 concerning the bill --- this from at least two justices (Scalia and Thomas) who routinely say that any use of legislative history is a sham because it's necessarily incomplete. One wonders what a quote not uttered on the floor of Congress but to a journalist would constitute in that case? In any event, rather than holding the mandate costitutional and those portions of the bill inextricably linked with it (guaranteed issue/community rating), four members of the Court were primed to throw the whole bill out. That level of judicial activism, in a context like this one, would be nearly unprecedented.
I imagine the dissenters either had Roberts's vote or that Roberts left the post argument conference without commiting to a side and saying something to the effect of "let me see how it writes." He certainly didn't trust the dissenters, as he clearly instructed his law clerks to begin working on an alternative majority opinion (the final product was too polished and too long to have been written at the last minute). And he waited to see what was written.
What was written was not measured judicial analysis, but rather an opinion that started with a goal --- throw the bill out --- and then figured out how to get there, blowing by any precedent in its path. The challengers were right in one respect, in that the mandate was a unique use of federal power that had not been considered by the Supreme Court. But severability had been considered by the Court literally dozens of times, and the four dissenters charged right by what those decisions had said.
So Roberts was left with a choice: engage in the severability analysis himself (a messy task indeed) or find some other way to uphold the bill. He chose the latter, and the result is what we have today.
That dissent intended to get his vote. It might have had it only struck a portion of the law. But Roberts correctly realized that he couldn't jump off that cliff without precedent or logic supporting him. Kennedy, Alito, Scalia, and Thomas went all in. And they lost their bet. Just like the conservatives in Congress.