The answer may be simple that the Earth's gravity affected the apple and the apple fell to Earth.
But from a cognitive point of view, this answer cannot be considered sufficient to answer a simple question like this because we didn't address why the apple was on the tree in the first place, and we didn't explain why the apple was heading towards the center of the Earth when it fell.
Assuming that we have decided to go deeper into the answer, we have explained the history of plant evolution, explained the photosynthetic mechanisms and diverse plant strategies for successful reproduction. And then we explained the existence of the apple as one of these strategies aimed at spreading apple seeds. Then we went into gravity and analyzed what atoms make up the planet and explained the relationship of mass to the temporal tissue and how this relationship results in a curvature of place that attracts more atoms to the center.
This might be a better answer to the original question, "Why did the apple fall?" But it's not the ultimate answer because we didn't explain why there was a mass of matter and we didn't explain the mechanism of action that results in the curvature of the spatial tissue.
Assuming we interpret the evolution of plants and the relationship of gravity to matter, we need to clarify more concepts, such as what atoms formed plants and how they evolved and explain the physical relationships that govern them.
Without answering these questions we can't say we gave a full answer to why the apple fell off the tree.
Well, then... Let's go back to the moment of the Big Bang, when it all started, and try to understand the steps that led to the emergence of a mass of matter, and analyze the energies of this expansion into the emerging phase of what we call the four cosmic forces...
That's the beauty of cognitive curiosity... No matter how simple it is, no matter how easy it is. We could sail all over the universe looking for answers. Intellectual suicide is to accept simple answers that do not alter our cognitive state and do not respect our computational ability to think. When I write some articles that aspire to explain natural phenomena, I don't ask everything that I learn in the research phase, because there's so much information and it's not directly related to the subject matter. For example, I'm writing an article on photosynthesis, but within the research I had a question. Where do the carbon atoms that make up the tree come from? It comes from carbon in the air in the atmosphere of the ball.