We can't identify the plant (tree? looks larger than a shrub) in the photo. Are you able to get a clearer picture in brighter lighting so we can see the foliage?

Hickory trees, like any shade tree, can be vulnerable to an array of potential causes of decline. Our "What Causes Trees and Shrubs to Die?" web page addresses many of the contributing factors. Often, stress initiates decline well before any symptoms become obvious. Stress could be due to drought, overly-wet roots and compacted or poorly-drained soil, cumulative exposure to too much ice-melting salt runoff that injured or killed roots, poor pruning cuts or storm damage breakage that developed wood decay that entered the trunk, and injuries to the trunk base.

You can always get a second or even third assessment from another arborist; make sure the person is either a certified arborist or licensed tree expert, as they have training in tree diagnosis and safety evaluation (some people have additional certifications in the latter). Even so, it can be difficult to diagnose a tree that has been in decline for some time, whose canopy is already substantially thinned or dead. Arborists can't treat or cure every condition, but they can help to narrow-down a diagnosis, and for trees not too far gone, also look for sources of tree stress that might be able to be alleviated.

Miri