On one level, of course, the Trump phenomenon simply represents an apparently more extreme, but logical development of his political party's electoral dilemma. If you are a party whose primary policy purpose is to rig the system to guarantee the continued enrichment of a small economic elite, more or less at the expense of everyone else, you need a strategy - and certain strategic issues - to appeal to others who do not necessarily stand to gain from your party's economic agenda but whose electoral support is essential for your party to achieve power. So you appeal and create an emotional connection on cultural kinds of issues - guns, religion, racism, xenophobia, etc. The Southern Democrats did something similar for nearly a century, using racist appeals and the emotional connection racism generated among whites of all classes to maintain the electoral loyalty of poor white voters - hence keeping the local elite's power secure.
For decades, Republican politicians have been appealing to the status insecurities of older, often downward mobile white Americans, many of whom are among the primary losers in our post-modern globalized economy. Racism and xenophobia have always been present in American society, but have been most potent in such periods of economic and cultural change. As the ethnic, racial, and religious composition of American society evolves, and as traditional norms and expectations are being rapidly undermined (more rapidly that perhaps anyone would have expected even 20 years ago), such divisive appeals are ever more effective with certain segments of the population.
In the past, political elites could manage such divisive rhetoric and rein it in when it became too threatening to social cohesion. Nowadays, however, such elites exercise less power, for several reasons, in particular the sorting out of the American population into separate ideological tribes with ideology-reinforcing, separate sources of news (and hence separate "facts") and the widespread explosion of divisive, fact-free, rhetoric in talk radio and social media. From flirting with "Birtherism," to demeaning and threatening immigrants, and now singling out Muslims world-wide as objects of hostility, Trumpism has liberated the most dysfunctional aspects of American political culture from the feeble remnants of elite restraints, leaving "establishment" elites looking not only powerless but rather ridiculous.
For they have sown the wind, and they shall reap the whirlwind (Hosea 8:7).