Here's how it works:
A Liberal justice approaches Justice Kennedy and says, "Say, my friend, keep this under your robe, I know you are going to vote against this, but if you give this same sex marriage case your approval, I promise that when you approach me in the future on a case that is equally important to you, I will side with you, even if I would vote to opposite way, cross my heart and hope to die. Really and truly, I will!"
So, sometime in the future, when Justice Kennedy desperately needs a NO vote on a case involving a snail darter eco case, he approaches the justice who made the promise and says: "Say, my good friend, you recall that you promised me a 5th vote on a case that was important to me if I would vote your way on that same sex marriage case back in 2015. . . well today's the day and I'm calling in that favor. I need you to vote NO on this snail darter case. Remember you promised and even crossed your heart and hoped to die."
The other justice says "WHAT? I don't know what you are talking about! How dare you suggest I would sell my vote on such an important case! HOW DARE YOU INSULT MY INTEGRITY!"
Or, alternately, Justice Kennedy did get someone in the past to vote with his side in exchange for a vote for something they'd want in the future. . . and this is his turn to payback that vote with a swing 5th vote.
Yes, if I was Justice Kennedy, I would definitely get my vote up front. Marxist have a lot of trouble remembering things they promise.
Why would a liberal justice need to approach Kennedy about his vote? Kennedy wrote the decision in last term's Windsor case. His view on same-sex marriage is quite well known as a result. Those who are surprised he sided with the majority today have simply not been paying attention.