There is many a Harry Potter fan theory out there, and some of them are completely insane. But every now and then a real gem emerges from the deep corners of the Internet, and this is definitely one of those times.
A Quora-user called Steve Jones has come up with a pretty decent theory that suggests the centaurs in the Forbidden Forest predicted Harry's destiny back when he was still in his first year at Hogwarts.
So how does it work?
Remember that first time Harry met one of the centaurs? It was in Harry Potter and the Sorcerer's Stone, during his visit to the Forbidden Forest, when the centaur Firenze chased Voldemort away. He carries Harry on his back, something the other centaurs are really unhappy about.
Now you need to remember that centaurs are excellent at divination, and they can read the night sky to foresee the future. And from what they tell Firenze, it seems as if they've seen something about Harry's future, and they don't want Firenze to tell him:
"What have you been telling him? Remember, Firenze, we are sworn not to set ourselves against the heavens. Have we not read what is to come in the movements of the planets?"
The fan theory goes that the centaur knew that Voldemort and Harry would meet in the forest and one of them would die. As we know, this happened in Harry's final year at Hogwarts, where Harry sacrificed his own life only to be resurrected again. (If you'd like to check out that scene, here's a convenient clip for you.)
And that's basically it; the centaurs knew about this way before it happened, but neglected to tell anyone because they don't want to interfere too much with humans. It's a pretty simple theory, but it does make sense and I'm sure most of us overlooked this possibility when we were reading the books. Whether or not it's true is hard to say, as J.K. Rowling would need to confirm or debunk it.