When a plant no longer needs a leaf it will let it die and fall off. As a plant flowers it will lose fan leaves that's natural because it used those to grow big and fast with but no longer needs them because it doesn't take as much energy to flower. There's microbes in the soil feeding the plant too. The plant keeps the leaves it needs to finish flowering but if they are kept green with ferts the plant builds up nitrite and nitrates so when it's harvested it's loaded with them and they're not good for you to smoke so if the plant is allowed to lose those leaves naturaly it will use the stored nitrites so your smoke isn't full of that shit.
Leaves die in fall off naturally, sure, but not because the plant says it's time.
Instead, as photosynthesis occurs, chlorophyll breaks down in a process called "photoexitation", where electrons are stolen from each pigment (including chlorophyll), and used in photo system 2, during the Calvin Cycle.
As leaves on the lower growth received less photons, the leaf cant maintain ATP levels to continue the Calvin Cycle, and chlorosis occurs. Part of this is that older growth has a lower sink strength, and the plant food makes it way to new growth (high sink strength) first.
The Calvin Cycle is like an engine. It uses most of the energy it makes to make more energy. Only small amounts of APT it creates, is used for G3P production, and later carbohydrates (sugar), and Photosynthate.