Thinning or dying crown
Especially when it starts at the top and works down.
Major diseases and insect infestations can kill a tree quickly — and spread to the healthy trees around it. Catching it early is most of the work.
A tree in decline looks much the same whatever the cause. Drought stress, root damage, a boring insect and a fungal pathogen can all produce the same thinning canopy — and the treatment for one does nothing for the others.
So the first job is finding out which it is. That means looking at the crown, the trunk, the root collar and the site, rather than spraying and hoping.

Especially when it starts at the top and works down.
Small exit holes and fine frass usually mean a boring insect already inside the wood.
Shelf fungi on the trunk or root collar indicate decay in the structural wood, not just a surface problem.
Dark weeping patches on the trunk are worth identifying rather than washing off.
Out-of-season colour or leaf drop is a stress response with many possible causes.
Several things spread tree to tree. If one nearby has died, the ones around it are worth checking.
Free estimate and a proper diagnosis — before it spreads to the ones next to it.