Here are just a few posts or articles from Jack Balkin, Sugar Mr. Poon, Michael Kinsley, NYT's Linda Greenhouse, Tony Mauro, Southern Appeal, the AP, US Newswire, Anne Gearan, the Fourteenth Circuit, Robert Prather, the Federalism Project, and the consensus is nobody knows why Justice Rehnquist joined with the majority. The best reason I've read stated here is that he wanted to keep the opinion under control. The second worst reason I've read, suggested here , is that he was horse-trading with Justice O'Connor for votes on the forthcoming race in admissions cases, and the very worst is because "Hibbs was about a white man suing a state," as stated here.
Since I'm reading this book on Ken Starr, I'd guess that Starr would say that the case is in some ways like Miranda, where the Court generalized about police practices in the various states without much of an actual record, and the Chief Justice's more recent vote to uphold Miranda, which ran against the grain of Rehnquist's views in prior cases. (That's right, I read one book and I think I'm an expert.)