What causes the top of my tree is dying?

The usual culprits are root injuries nobody connected to the tree: a driveway poured, a utility trench cut, a patio graded, fill soil spread over the root zone, or years of compaction from parking and mowing on wet clay. Add drought stress, soil that stays saturated after heavy rain, and girdling roots circling the trunk from a nursery pot planted a decade ago. Then the specific killers, which usually arrive after the stress rather than before it: vascular wilts and wood-boring insects. In ash, upper-crown dieback with sprouting below has meant emerald ash borer for years now, and that particular outcome runs on a different and much less forgiving timeline than the rest.