It may be September, but it isn't fall yet. So why are some of your trees already dropping leaves? There are three main reasons. Let's discuss each scenario from no problem to minor problem to major, big-honking problem.
No Problem -- It's the Weather
Shade trees typically grow lots of leaves when the weather is pleasantly warm and they're getting plenty of rain to keep the juices flowing. But growing lots of leaves puts a huge burden on a big tree that can be hard to maintain.
So when the weather gets really hot and dry in summer, the tree responds by dropping some leaves. The tree figures autumn will soon be here anyway, so what's the big deal? Tulip poplar (Liriodendron tulipifera) is notorious for doing this. But the good news is it doesn't hurt the tree.
Minor Problem -- Fungus or Bug
Some trees tend to get leaf fungus in late summer that leads to leaf drop. Grumpy thinks part of the reason is that the tree just gets tired of using its natural chemical defenses. For example, every August and September, no matter how wet or dry the weather, my 'Sioux' crepe myrtle gets leaf spot. One by one, the leaves turn red and drop, until the tree stands nearly naked with pink blooms still on the top. It's gotten too big to spray with fungicide, but since this annual leaf drop doesn't hurt the tree's health, I don't care.
The leaf drop culprit could also be an insect. In the case of my sugar maple pictured at top, the little jerk responsible is a bug called the maple petiole borer. This tiny sawfly lays an egg in the base of the leaf stem, called the petiole. A larva hatches out and burrows into the petiole, interfering with the transport of water and nutrients. Leaves often drop off green. Fortunately, not enough leaves fall to hurt the tree, so I don't spray.
Major Big-Honking Problem -- Sudden, Total Leaf Drop One day in early September, your oak tree looks fine. Suddenly, the leaves turn light green, then yellowish, then brown, and then they all fall off. This might by OK in Montana, where fall comes early, but not in the South. While there is a slim chance your tree will leaf out again next spring, if it's brown or naked while all the trees around it are still green, chances are it just croaked. If it doesn't leaf out next spring, cut it down. No tree skips a year of growth. No way, no how.