Let's assume Trump runs a shrewd campaign. Now, combine that campaign with even one of these exogenous shocks:
1) A severe and successful "9/11" type attack on the mainland United States (the attack might be conventional, nuclear, biological, chemical and/or cyber -- but severe in effect) occurs close to the election. In the resulting chaos and panic, the GOP (rightly or wrongly) paints the Democrats as soft on terrorism and/or national security -- Trump becomes the man of the hour.
2) Assuming Clinton is the Democratic nominee, she might be indicted as a consequence of her private email server and/or the activities of her family's foundation, or perhaps key members of her team will be charged with criminal offences. (I realize for people on the far right, it is a matter of religious belief that Clinton is guilty, and for people on the far left, it is an equally fervent belief that she is innocent. As a more neutral observer, I will opine that it's possible a crime -- or crimes -- may have been committed.) An indictment could occur in the middle of the election. Remember, one of Clinton's greatest defects is that she's viewed as untrustworthy (some polls already show that about 60 percent of Americans feel she can't be trusted). If criminal charges are filed against Clinton or her team, the impact on her campaign would be enormous (one might even say "Yuge") -- and that might tilt the election to Trump.
3)An economic collapse on the scale of the 2008 financial crisis begins (perhaps triggered by China or the technology sector). At this point in the 2008 election cycle, the unemployment rate was 4.9%, and by November 2008 it had risen to almost seven percent. Imagine the impact that kind of economic debacle would have on a Democratic presidential candidate's prospects.
4) Clinton (age 6
or Sanders (age 74) -- whichever is the Democratic nominee -- dies (from natural causes, a plane crash or whatever) or is incapacitated (e.g., by a severe stroke), and the Democratic Party finds it difficult to rally around a new and strong candidate. The GOP candidate would likely benefit from the resulting chaos.
5) Michael R. Bloomberg (or someone else) runs as an independent, and the election results in no presidential candidate winning a majority of votes in the the Electoral College. The presidential election would then be decided by the House of Representatives. (The voting process is by state delegation, and you can find the details here.) The bottom line is that the GOP would probably decide on the next President, and they would likely vote for the GOP candidate -- Trump.