This does not look like normal needle shed.  There may be several reasons for yellowing foliage and needle drop.  In general hemlocks decline due to our heat and humidity and just do not grow that well here.  They grow best in cooler locations. Other issues include poor drainage, poor site conditions, and compacted soils. 

When stressed the yellowing foliage may indicate possible infestation by insect pests such as scale or woolly adelgid.
https://extension.umd.edu/hgic/topics/scale-insects-hemlock
https://extension.umd.edu/hgic/topics/hemlock-woolly-adelgid-trees

The tree looks like it will continue to decline and you may have to consider removal. Outside of consulting an arborist, there is nothing you can do for now. If you can reach the lower foliage, inspect it for pests on the undersides of the branches; scale and adelgids tend to feed on foliage or along the stem itself. Keep monitoring the root zone for watering needs as we progress through autumn, as we have experienced drought in autumns past (just last year, in fact). Soak well when needed (as the soil dries several inches down) but otherwise avoid keeping them soggy for extended periods.
Here is information on when to decide to remove a tree https://extension.umd.edu/hgic/topics/how-do-you-decide-when-remove-tree

You can search for an arborist using the second and third links on the above page .

Marian

Thanks very much!  I really appreciate your feedback!

You are welcome.

Marian