Answer: It is so difficult to determine the cause of problems like these! My first thought is usually water-related: either too much or too little. This is a common problem with young plantings.
Is the tree in full sun? If the lower branches are shaded, that could be a factor.
I would examine the twigs carefully, looking for signs of insects or disease. You might contact a local arborist and see if you can bring in a sample, or have them come out to evaluate the tree. If this is a prized specimen, it might be worth the expense.
Sorry I can't be more definitive.
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