There have been a number of recent reports on how and why the Supreme Court moved left this past term. Two notable pieces were this one in the New York Times and this one by Tom Goldstein at the Scotus Blog. A common explanation is that the conservative justices have begun to split much more than the liberal ones. I am not sure that I agree.
In my view, the main explanation is that, in this past term, Justice Kennedy simply voted with the liberals more often than with the conservatives. If I am right, then there is no elaborate explanation that is necessary. The results are simply what Justice Kennedy preferred, based presumably on the constellation of cases that arose.
There are different ways to analyze the statistics, but here is the basis of my argument. (I based my statistics on this page from Scotus Blog.) There were 19 5-4 decisions this past term. I excluded 3 of them, because the majority in these cases involved either two conservatives and two liberals, or three liberals and two conservatives. In those cases, the decision was arguably not based primarily on partisan differences.
That left 16 5-4 decisions where the difference was primarily partisan. In 13 of these cases, Kennedy was the deciding vote, either joining the 4 liberals or the 4 conservatives. In 8 of these cases, Kennedy joined the liberals; in 5, he joined the conservatives. Thus, Kennedy dominated the Court, once again, and this term he leaned towards the left.
In the remaining 3 cases, Kennedy was not the deciding vote. In two of these cases, a conservative joined the four liberals (Roberts once, Thomas once); in one of these cases, a liberal joined the four conservatives (Breyer). It is true that this suggests that the conservatives splintered a little more than the liberals, but not by much.
I am not arguing that this is the only way to analyze the term. In particular, I leave out of my analysis 6-3 decisions, such as the Obamacare case of King v. Burwell where both Kennedy and Roberts joined the liberals. My point, though, is that we should not be too quick to find new trends, when there is a perfectly reasonable way of reading the statistics as showing that, at the Supreme Court at least, Kennedy remains King.