Answer: Based on your description, I suspect it is drought related. Although you have been watering it, it is possible the water has not been penetrating deeply enough to deep water the roots. You might want to dig down and see how far the water has penetrated, sometimes it is surprising. (Ideally, water, wait a few hours or overnight, then check.) You would want to try to water deeply less often rather than sprinkle lightly more frequently. The deeper watering encourages deeper rooting to where the soil can stay moister longer naturally. To know when to water, it is a good idea to dig down and see; when the soil is dry about three inches down, then you would water -- for a tree that has been planted for such a long time. Using several inches of organic mulch over the root area also helps to keep the soil moister longer.
Another possibility is that there was some other type of stress such as accidental herbicide drift or an adverse reaction to a chemical spray applied in hot weather, or something of that sort has happened.
This late in the season, the leaves have for the most part accomplished their job of supplying the tree for the year, so the defoliation is not necessarily fatal. A tree will sometimes defoliate in response to drought, going essentially into drought induced dormancy as a self protective measure.
You might want to consult with your arborist, your county extension, and/or possibly your local professional nursery personnel for a more specific diagnosis and suggestions as to how to proceed. I'm sorry about your tree.
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