Answer: I really can't diagnose a problem without seeing the tree, so I can't tell you why the leaves and/or branches might be dying. Insect infestations and diseases can cause such problems, as can root stress from overly wet and poor draining soils. Since the damage is only on the sunny side of the tree, the leaves may be showing symptoms of sunscald - but this kind of damage should affect all of the leaves on one side, not just the lower branches. You may want to simply prune off the lower branches to make the tree look more symetrical and to remove the damage. Sorry I don't have other suggestions to share.
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