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The leaves can fall off your smoke tree for several reasons:
too much, or too little water
insect or disease damage
damage to plant by weed-eaters or mowers
age of tree
too acidic soil

This article should help: https://www.gardeningknowhow.com/ornamental/trees/smoke-tree/growing-smoke-trees.htm

As long as you don't cut back more than a third of its growth in one given year, it should be fine. This article will help with pruning your smoke tree: https://www.gardeningknowhow.com/ornamental/trees/smoke-tree/pruning-smoke-trees.htm

When plants don't flower as they're expected to, look to sun (which you say this plant has plenty of), or soil. Here's an article about smoke trees: https://www.gardeningknowhow.com/ornamental/trees/smoke-tree/growing-smoke-trees.htm
As the article mentions, the smoke tree likes alkaline soil, so you might want to check the pH of the area where the tree is, or have it checked A nearby extension office https://www.gardeningknowhow.com/extension-search/ can help you with that.
Also to encourage flowering, you could add some high phosphorous fertilizer, or some bone meal

It is probably a fungus, though there are so many, it is hard to say which one. Cut the affected branches off and treat the tree with a fungicide to kill off any residual fungus on the tree.

Smoke trees are so pretty. Here's an article to help you double check your care: https://www.gardeningknowhow.com/ornamental/shrubs/barberry/barberry-shrub-care-tips.htm
Also, here's a list of deer resistant plants for Georgia. http://www.caes.uga.edu/extension/dekalb/documents/HGT-DeerResistantPlants.pdf
As you can see, smoke tree is on the list.