There is also an opinion kicking around in the media, speculating that a die-hard conservative staffer leaked it. The logic is something like this: The opinion was supposed to be still a secret, circulating internally among the justices for further debate and perhaps modification. There is some chance that a more moderate Repub on the court (read: Roberts) could get them to water it down a bit before they release it.
By leaking this hard-nosed hard-right opinion now in its current form, it put any of the conservative justices in a very awkward position if they want to amend the decision to water it down before they release it. The thinking is, by leaking it now, it works towards getting it cast in stone just as it currently stands.
See the section of this article on the idea that a conservative staffer may have leaked it: