Big Holly Tree top appears to be dying
Dying at the top usually indicates a root problem. Then the tree can't get water and nutrients up to its farthest reaches, i.e. the top.
We can't say what the cause would be. You are welcome to send us a photo of the tree.
Examine the base and trunk for damage or signs of insects or disease. Has anything changed in its vicinity? We would need more information to be able to diagnose the issue.