First Theory: Those who live by the sword, die by the sword. Think of how many people Trump has pubicly called "losers". Now, he's on the verge of perhaps being the biggest loser ever in Presidential elections in US history. That's got to create a HUGE amount of cognitive dissonance for him. So I would expect him to become increasingly irrational in trying to resolve it.
Second Theory: I like to say that Republicans, and now Trump, engage in what I call the "Pot caling the kettle black strategy". They accuse the other side of doing exactly what they are doing, do it first and do it often. i.e. blaming Obama for worse race relations in he country; Trump calling Hillary a liar. So when they've made such a big deal about voter fraud on the Democratic side, I fully expect them to enage in election fraud, i.e. tampering with voting machines.
When I wrestled in HS, our coach taught us to push on someone so that when they pushed back, we could use their own actions against them to take them down.
Trump is claiming the elections will be rigged against him. So he's got everyone saying "No" and that the people should have complete faith in the election results, and has Hillary on record saying that the loser should accept his/her fate without question for the sake of our democracy. Suppose Republicans, who control elections in 30 states, do engage in election fraud, and Trump pulls a "Bush 2000" and wins, defying all the polls. They've already offered the theory that there's a lot of closet Trump voters out there to explain such an event. But now, they've also gotten everyone, including Hillary to say the loser shouldn't question the integrity of the vote, and should gracefully concede.
It's just like what my wrestling coach taught us to do. Push on someone, and when they push back, use their actions against them to take them down.