I've posted many theories, but I've never actually created a theory myself. Until now.
Today, I am going to tell you my theory (which might not be true, but I feel is plausible) that the events of Back to the Future II & III never actually happened and it was all a dream.
At the end of the original Back to the Future, Marty uses the lightning from the clock tower to travel back to 1985. Doc drives Marty to his house and Marty goes to sleep. When he wakes up in the morning, he sees that his mother, Lorraine, was now happier and healthier, Biff was nice and no longer George's supervisor, and George was confident and a writer. But I see one thing that doesn't make sense. Look at George at the beginning of the movie and George at the end of the movie.
Why does he look so different?
I mean, we know Lorraine looks different because she stopped drinking and smoking.
And Biff looks pretty much the same.
But why does George look so different?
Marty later discovers a 4x4 truck in the garage that belonged to him. Jennifer arrives, and then Doc. Doc takes Marty and Jennifer to October 21, 2015 at 4:29 p.m. There, we see old Lorraine, old Biff, old George, and ...old Marty? Old Jennifer? Their kids?
Here's the problem: Marty and Jennifer travel from 1985 to 2015, meaning that they're skipping 30 years of their lives. They're vanishing from 1985 and reappearing in 2015, meaning that there's no Marty and Jennifer in 1985 to grow up and have kids, meaning that that should not have happened.
Now, in dreams, the events of the dreams usually contain you, the people you know, the places you've been to, and the things you've seen/experienced. But they usually don't make logical sense, which is why George looks so different when there's no reason why his physical appearance shouldn't be the same as in the beginning of the original film and why Marty, Jennifer, and his kids exist in 2015.
After Marty fixes the future, he buys a sports almanac covering sporting events from 1950-2000, and plans to bet on the winner and be rich. Doc throws the book away because he didn't invent the time machine for personal gain. However, old Biff overhears this. Later, he steals the DeLorean, goes to 1955, gives the almanac to his past self, and goes back to the same 2015 it came from.
Here's another problem: Biff altered 1955, so he should have traveled to a 2015 where Biff was rich, powerful, and Donald Trump. Instead, he brings it to the same 2015 he came from so that Marty can still have the DeLorean in that year and this way, he can go to 1985.
Marty and Doc travel to the 1985 with Donald Trump and they then discover that Biff had given the almanac to his past self on November 12, 1955, the EXACT same date that Marty had gone to in Back to the Future. They go back to there and if you watch videos like this, you'll see that the Enchantment Under the Sea dance scenes in the two movies were not completely in-sync (ex. when Marvin calls his cousin).
Eventually, Marty gets the almanac, burns it, and fixes the future. But then, the DeLorean gets struck by lightning, therefore sending Doc to 1885, right?
Doc should have died. The DeLorean can only time travel when it's traveling at 88 mph and the Flux-Capacitor has 1.21 gigawatts of energy, powered by a machine called Mr. Fusion, but can also be supplied by plutonium or a bolt of lightning. And the car was struck by lightning, therefore sending 1.21 gigawatts into the Flux-Capacitor. But it has to be traveling at 88 mph, too. Doc had the car completely still in mid-air, so the DeLorean should have been burned into a crisp, and Doc should have been, too.
1955 Doc and Marty discover Doc's gravestone. Doc had died on September 7, 1885 after getting killed by Buford Tannen over a matter of $80. Marty travels to 1885 using the DeLorean Doc had stored in the cave for 70 years, planning to save Doc from getting killed. However, the fuel line ripped once Marty got to 1885 and without gasoline, they couldn't get the DeLorean to 88 mph (so NOW the rule applies).
Buford tries to shoot Doc, but Marty throws a pie plate at his hand just when he fires, causing Buford to misfire, saving Doc's life. After some punches are thrown, Buford gets a face full of manure and gets arrested. They use a train to push the DeLorean to 88 mph, and Marty is sent back to 1985.
Doc makes it to 1985 with a train he made into a time machine. This doesn't make sense, either.
Apparently, in the 1800's, Doc was able to get the supplies he needed to build a working Flux-Capacitor and time dial. He was able to do a bunch of other stuff to the train, too in order to make it work. AND IT WAS THE 1800's. Marty probably had this dream the night he got back to 1985 after all that time travel he just did.
So, that's the theory. I hope you liked it. Please comment and follow my account for more theories and fun facts! :)