Answer: Top dieback is usually a signe of a borer infestation. Other sypmtoms are "sawdust" or holes visible in the wood. Also check for swellings, cracks, or other symptoms of damage/disease that might interfere with the movement of water to the top of the tree. You'll need to remove the borer/canker/gall from the tree and train a new branch to a stake, so it can become the central leader of the tree. If none of these ideas cover your situation, have a professional consultation.
It may also be that the tree roots are outgrowing the space they're in, but that usually causes general dieback of all branches, not just the top. Best of luck!
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