The most obvious answer for what's driving the change is same-sex marriage and marijuana. Between 1999 and 2015, recreational marijuana and same-sex marriage have gone from being legal in zero states to legal in four and 36, respectively (plus the District of Columbia).
And not coincidentally, that's come with significant uptick in public support. Below is Gallup data for public opinion on five social issues, along with the percentage who said they were "conservative" or "liberal" (the red and blue lines) between 2010 to 2015 (not all issues have updated 2015 data yet).
The graph shows that a large number of Americans might feel "liberal" on certain social issues but don't necessarily consider themselves socially liberal or conservative overall. Support for stricter gun control, legalizing marijuana and same-sex marriage all cracked 50 percent at some point in the 2010s, but those who identify as socially conservative or liberal hasn't gotten above 40 percent. Those red and blue lines are near the bottom of the graph.
In case that chart is a little hard to read, here's the net change in support for the five issues from 2010 to 2014. Everything is up -- especially same-sex marriage.
That shift toward social liberalism has been accompanied by an uptick in overall liberalism too. Gallup earlier this year showed a new high in self-described "liberals" in the United States -- even as it still trailed significantly behind the "conservative" label. In 2014, 24 percent of people used the L-word to describe themselves -- up from a low of 16 percent in the mid-1990s.
So how do we know it's because of these social issues and not other ones?
Well, the number of Americans who view themselves as liberal when it comes to economic issues quite simply hasn't risen accordingly -- which goes to show why so many people like to say they are "socially liberal but fiscally conservative." Social liberalism is now quite a bit more acceptable to people than economic liberalism.
There are certainly other factors that could be at play, up to and including foreign policy. But the most obvious answers -- and ones that makes a lot of sense because they have moved so quickly -- are gay marriage and marijuana.