Cost Benefit Analysis

Leaves have a job to do. In the economy of the plant they are the producers. Chloroplast in the plant harness photons from the sun to turn water, acquired through the roots, and carbon dioxide, obtained from the atmosphere, into sugar. Sugar is molecular gold for the plant because it can be broken down to be used as energy or repackaged and assembled into more complex carbohydrates like cellulose and starch.

However when the economy is down due to shortages in natural resources the tree has to make some savvy business decisions.  The most productive leaves are those that get the most sunlight, the ones on the outermost part of the canopy. The central leaves also produce but with much less efficiency. When times are good there is more than enough resources to be processed by the efficient and less efficient leaves. But when resources are scarce the tree closes down its least effective factories and reallocates the available raw materials toward the best producers. 

Then, like any factory that becomes unoccupied, the vacancy leads to disrepair and in the end the leaves fall in the middle of summer rather than fall in the fall when they should.