* I can think of two scenarios that would fit the bill. One, Trump faces Clinton and, late in the race, something happens to render Clinton unelectable. Two, Trump faces Sanders and Bloomberg jumps in, splitting the left vote and throwing the election to Trump.
Both seem highly unlikely to me, the first because Clinton is already the most intensely vetted figure in US politics, the second because Sanders is unlikely to win the primary.
3rd circumstance: another financial crash. These are always bad for incumbent parties in an election year. Even if it's because of global conditions, or the idiocy of the Republican congress in blocking regulation, it hurts the party who holds the presidency. And Trump would say he's an outsider with business skills who would have stopped it, and knows how to fix it. That's bullshit, but a lot of people might fall for it.
How unlikely a crash is, I can't tell. Given the financial news of the past few months, I don't think it's
highly unlikely. And that's why I don't want Trump to win the Republican nomination, just in case something happens to make a change of party look like the main thing voters want.