Like I said, I think it was the change in society. We've become aggressively intolerant which is manifesting in various ways. Do you recall during the primaries when Trump supporters were literally being assaulted and hounded, just for being Trump supporters? Or how the media was running, non ironically, pieces literally comparing Trump to Hitler. The sort of stuff you might expect from internet trolls, not actual reporters.

Compare this to, for instance, how the media and society society responded to e.g. Romney, McCain, or Bush. Society has shifted sharply towards intolerance which is making obtaining accurate information very difficult when that information might leave one within the crosshairs of said intolerance. This creates a major chilling effect which leads people to misrepresent their views. Take the state of society today and then imagine somebody calls you and asks if you're voting and if so, for who. You're going to get some decent chunk of Trump supporters that will lie here. Voting is anonymous, but phone calls are not.

The burden of somehow trying to overcome this problem is on pollsters, and the only thing that's certain is that it's not going to be easy. Gallup was way ahead of the game here -- deciding for the first time ever to not make predictions about the 2016 election.