This is the basic shape of the material forces at work in contemporary politics. They are, incidentally, the same dynamics at work in every administration that has failed to confront capitalism: liberals try to finesse it with increasingly futile welfare and regulatory measures, reactionaries double-down into increasingly rabid bigotry, but both are caught in fascism's pincer. The explanatory power of a materialist understanding of history is not that it can predict every incident and micro-trend that emerges within this framework: historical materialism allows us to understand the cumulative trajectory of all of these different moments, and to anticipate the choice between socialism and barbarism that they will, with increasing force, impose upon our politics.
So let's return, then, to the specific question of Trump. Historical materialism does tell us that, pending a real confrontation with capital, we are increasingly likely to see reactionaries take power - but that does not mean that Trump was destined to win the Republican primary, or that the Republican was destined to win the general election. Historical materialism does tell us that reactionaries will try to channel class anxieties towards various sociocultural scapegoats - towards constructed identities like race, gender, and nation - but this channeling need not occur in any particular way, so long as it happens. One way you can do this is to fixate the reactionary id on building a giant wall against immigrants; but as we have seen in the past, you can also do this by ginning up resentment against the racist image of an indolent welfare queen, or against the devious careerism of the affirmative action beneficiary, or against the anti-Semitic figure of the parasitical Jew.
And indeed, we have seen all of these currents at work in Trump's base - nothing about historical materialism predicts that he would need to focus his politics on any one of them in particular. Nor does it predict that he would need to rely, in particular, on the tactic of a government shutdown in pursuit of that agenda; you can play wall-politics without that, just as he has until now. Nor, moreover, does it predict that he would draw out that shutdown for 35 consecutive days - another tactical choice that might have easily gone another way.
From a materialist perspective, the most we can really say about this shutdown is that capitalism makes the election of reactionaries likely, and makes it likely that they will try to channel class anxiety into a reactionary agenda, and that opposition to these trends will increasingly take the form of class warfare (like labor strikes). But historical materialism is not a crystal ball that lets us predict or explain every little detail of our world within infinite precision and clarity; if you want to understand why the shutdown lasted as long as it did, then it probably makes more sense to look at things like psychology and Trump's ego-entanglement in an escalation of commitment. For the socialist, understanding the explanatory boundaries of historical materialism is just as important as understanding its potential.