According to physics, in addition to the force of gravity, the force of air resistance also affects the fall of an object, so when leaves fall, they fall in a floating manner, not as fast as other objects

I guess the reason why the leaves fall backward is because the front side of the leaf is heavier, and so the force of gravity is stronger on the front side than on the back side. The force of gravity on the front side of the leaf is greater than the force of gravity on the back side of the leaf

The air resistance on both sides of the leaf is the same, so the front side of the leaf is facing down and the back side of the leaf is facing up

And in biology, the front side of the leaf is the front side of the leaf which is subjected to the photochemical process

I looked up some information

The fenestrated tissues in the chloroplastic tissue of the leaf are connected to the upper epidermis, and the spongy tissues in the chloroplastic tissue of the leaf are connected to the lower epidermis. The fenestrated tissue has neatly arranged and tightly packed chloroplasts, while the spongy tissue has loosely arranged chloroplasts. Under light conditions the fenestrated tissue makes more organic matter than the spongy tissue.