It may be autumn but not all leaf drop is a good thing. Some trees simply aren’t designed to lose their leaves -or very few at least. Evergreens will still drop their leaves occasionally but it’s really when healthy, vigorous trees start to shed their foliage when they shouldn’t, that we should begin to worry.
Any number of problems can cause a tree to start dropping its leaves but by far the most common reason is stress. It could be due to a lack of water, too much water, irregular watering, pests, disease or even a change in soil level around its trunk, but one common variable in all of this is that stress is being placed on the tree and is almost certainly the underlying reason behind unexplained leaf drop.