Bob Davis asks that question. I can think of a few hypotheses, none well-grounded:
1. It was first necessary for America to recover from recession, so people could be less scared, thus feeling sufficiently secure to go a bit crazy.
2. Rising expectations are required to sustain a backlash, and finally the economy was strong enough to deliver some of those. This mechanism was discussed by Tocqueville in his book on the French Ancien Regime. Of course this is a close cousin of #1.
3. Obama actually has been a towering and calming presence. But after him…the deluge.
4. The “Great Man Theory of Trump.” He has unique skills, and an unusual celebrity background, and the relevant variable is when he chose to actually run for President.
5. The institutional and intellectual capital of the Republican Party was finally run totally into the ground. (But when exactly? And who perceived it as such other than Democrats?)
6. Americans have been paying closer attention to the terror attacks and refugee crisis in Europe than we traditionally might think, and thus they feel that the American system requires a radical wake-up call.
7. Traditional white males approached some kind of threshold where they realized from now on they will lose all political battles unless kind of radical rebellion is undertaken. This hypothesis reminds me somewhat of the South’s decision to secede shortly before the Civil War.
8. Social media are more potent, and that helps populist sentiment, but populism isn’t actually any more popular these days (see Krugman, who notes Obama has fairly high approval ratings).
These are just food for thought, I am not endorsing any of them. And for the most part they are not mutually exclusive.