A short answer is that both thrive when suitable fuel is available. In the case of wildfires, it is super dry forests and underbrush fanned by winds, some of which are created by the fire; in the case of Trump, the spark is lit by a demagogue, and the fuel is the mass of workers and their families displaced by jobs due to technology, while the top one percent make enormous financial gains.
Nine months ago, few if any thought that Trump could win the Republican nomination, and if he did that he could win the election. Now he has done the first, and some pundits, not wishing to be so wrong again, suggest that he could win the presidency. Not only is there a core of Republican primary voters that fuel his support, but there are Democratic backers of Bernie Sanders who feel disadvantaged in many of the same ways as Trump’s supporters. Could some of these defect and support Trump? Could the anti-Trump Republicans just not vote?
What will happen is anyone’s guess, depending also on who controls the Senate and House, and who gets to be the Supreme Court appointee(s). It was less than a hundred years ago that Mussolini and Hitler came to power through a democratic process and then assumed dictatorial powers. Stalin and Mao took control using a slightly different route. There is no shortage of demagogues seizing power when the conditions are right, and no shortage of forest fires when the fuel is available. It would be a mistake today to focus on the person and not the conditions that allow the person to attract supporters.