The best, and most concise, explanation I’ve seen yet. From the real life section of the trope page “Didn’t see that coming”:
Donald Trump in the 2016 Republican Primary was expected to be a repeat of the quickly-surging-then-tanking Hermain Cain from 2012 * as it turned out, the Herman Cain analogue turned out to be Ben Carson (ironically, also the only black candidate in that field and a non-politician), who did surge very highly (at one point in late October/early November 2015, his polling numbers were above Trump) but then Carson encountered his own version of Didn’t See That Coming when the Nov. 2015 Paris terror attack put national security & foreign policy at the top of the agenda, a field he was extremely weak in (there were reports he needed tutors just to coach him on the basics of foreign policy), and Carson’s campaign almost completely collapsed, going from around 25-30% to the single digits, but instead the Donald turned into something of an Outside-Context Problem for the entire Republican establishment, who were so used to traditional politics they had absolutely no idea how to campaign against someone like Trump. Not only did he have a better idea of how to manipulate the media for more coverage than anyone else in the field, he freely admitted to changing his mind on positions (defusing many attempts to characterize him as a “flip-flopper” with his perceived honesty), and owned his elitism instead of hiding it. Nor could donors be counted on to rein him in since he was rich enough to fund his campaign himself (and other donors were afraid of the pushback they might get from him and his supporters for opposing him so blatantly). And without a political past, opponents had trouble finding attacks that would stick.
Ted Cruz, one of the other Republicans, got waylaid by this trope as well. In the early going, the well-funded Cruz, who enjoyed strong support from movement conservatives for his principled stands despite those very same stands having alienated his fellow Republicans in Congress, figured Trump would quit as soon as he’d gotten his vanity fix, leaving a large bloc of voters there for the taking. Unfortunately for Cruz, Trump didn’t and began racking up primary victories. Cruz won his share, staying in the race even as almost every other candidate dropped out, but had neglected to have a backup plan and started pinning his hopes of winning the nomination on procedural maneuvering at the convention, putting him in an uneasy alliance with the party establishment he’d originally positioned himself as running against. These actually increased support for Trump, and Cruz dropped out with a month left to go in the primaries.
That is shockingly close to dead on accurate.
Their section on Clinton is worth reading as well.