As far as fan theories go, this is a bit of a weird one. But considering it's the 30th anniversary of Back to the Future, there's probably never been a better time to confuse ourselves with one viewer's odd and 'unique' take on the events of the film franchise.
According to Reddit user ducducguz, Back to the Future is actually the story of how a 4th-dimensional being essentially possesses the DeLorean and then reproduces itself. Do to this, it turns Marty and Doc Brown into unwitting pawns in its machinations and manipulates the events of all three movies.
It's a long and winding theory which covers a lot of bases, but let's take a look at some of the more important beats:
A 4th-Dimensional Being Gave Doc Brown the Idea for the Flux Capacitor
According to ducducguz, a 4th-dimensional being was able to communicate with Doc Brown via his failed mind-reading device.
In order to be given a physical, or 3rd-dimensional form, the being needed a flux capacitor, and so it planted the idea in Doc Brown's head. Eventually, through the creation of the capacitor and the DeLorean, the 4th-dimensional being gets a physical form - essentially meaning the DeLorean is sentient.
The Being Manipulates Events to Benefit Itself
The theory continues by claiming that in reality, Marty and Doc Brown are not actually in as much danger as it initially seems. Instead, the DeLorean entity is manipulating events to force Marty and Doc Brown down a course which benefits its ultimate goal - reproduction. Do do this, it changed the perception of certain objects, such as the fading photo, the newspaper and fax from Back to the he Future II and the photo of the Doc's gravestone in Back to the Future III. Ducducguz continues
But why did Marty start to disappear at the end of BTTF 1? I don't think he was in any danger of actually disappearing. The DeLorean entity needed Marty to believe that he was in danger of removing himself from existence to reinforce his belief that these manipulated objects reflect changes to the timeline. So it chose the moment at the dance to make Marty's hand vanish, because the moment with George, Lorraine, and the boy who tries to steal her most obviously illustrated the causality of the two ending up together or not. Again, in BTTF 2 Doc explains that they are removed from the timeline and unaffected by changes to it. But in this moment, the DeLorean entity reinforces the idea that Fox-Marty's existence is in danger because Fox-Marty is the one constant throughout the series, and his faith in this process drives him to influence Doc at the various critical points of their interaction.
The DeLorean Needs to Get to Doc to 1885
The final major point of the theory states that the DeLorean needed to get Doc Brown to 1885 in order for him to create another time machine.
However, it sensed that Doc Brown was becoming tired to time travel and would likely need to be manipulated in order to accomplish this task. To do this it sends Doc Brown to 1885, knowing he would eventually meet and fall in love with Clara. The Doc knows that he cannot remain in 1885, as his and Clara's interaction would result in changing history and the future and so the entity essentially forces Doc Brown to create the Time Train, in order for him and Clara to remain together. DucDucguz explains:
So the DeLorean entity chose to send Doc to 1885, where he would meet and fall in love with Clara, a woman of science and imagination who was supposed to have died tragically. It should have been obvious to him that she and him could not be allowed to remain in the past, or else they would have changed history so much through their everyday interactions that the Hill Valley of 1955 and 1985 would be unrecognizable. Not just through their offspring and descendants, but through Clara's continued teaching of the children of Hill Valley, and Doc's inventions and work as a blacksmith: the two would have been at the center of a lot of peoples' lives.
But the DeLorean entity blinded him to this fact so that Doc would be forced to break it off with her, leading up to him saving her on the hoverboard, stranding him in the past. After Marty successfully traveled back to the future, the DeLorean entity would have allowed Doc to realize his error, and a newly energized scientist would go to work on creating his second time-machine, with a second, brand new Flux Capacitor.
Ultimately, the creation of a second Flux Capacitor was the goal of the DeLorean entity as it helped to create a new 4th-dimensional being. As their reward for unwillingly going along with all this, the DeLorean improves the lives of its underlings - providing a partner for Doc Brown and cleaning up the life of Marty. In the end, everyone is happy, even though the DeLorean is destroyed. Ducducguz concludes:
The DeLorean was destroyed, but the 4th-dimensional being lived on, no longer needing to manipulate the lives of these primitive animals.
So, yeah. It's kind of weird, right? But, having said that, it does generally fit into the storyline, primarily because an inanimate object like the DeLorean cannot really behave contrary to a theory which suggests it sentient.
In any case, maybe you should use this bizarre theory as an excuse to rewatch the Back to the Future Trilogy for its 30th Anniversary. As if you even needed an excuse...
If you want to find out even more details about this theory, Make sure to check of ducducguz's original post here.