Andrew Koppelman suspects it was the conservatives:

My guess is that the court agreed to hear the California case because the four conservative members — Antonin Scalia, Clarence Thomas, Samuel Alito and Chief Justice John Roberts — hoped to overrule the lower court’s decision that the state had to recognize such marriages. It only takes four votes to put a case on the court’s docket, and they may be hoping that they can win Kennedy’s vote, while discounting the danger that he will go the other way. This would be a sensible call, since it is far from clear that any of the liberal members of the court are now willing to hold that same-sex couples have a right to marry.

Yet Kennedy could foil this strategy by declining, in the end, to review the case. Four votes are all it takes to hear a case, but five votes are sufficient to then dismiss that decision as improvidently granted.

Jeffrey Rosen is also focused on Kennedy:

In the most hotly contested previous cases, he has voted with the conservatives to strike down laws on the most sweeping possible grounds—such as his holding in the Citizens United case that corporations are persons, and his vote to strike the Affordable Care Act as a violation of Congress’s power to regulate interstate commerce. But Justice Kennedy is also deeply attached to the previous jurisprudence of … Justice Kennedy. Because his earlier cases give him and the liberals an easy way to strike down the Defense of Marriage Act and California’s Proposition 8 without resolving the status of gay marriage across the nation, there’s no reason to expect he won’t at least go that far. If Kennedy does write a narrow opinion, it’s not beyond the realm of possibility that he could attract a vote like that of Chief Justice John Roberts, who might not want to be on the wrong side of history.