Arborvitae Thuja Die Out & What to Do

Newly planted, within 12 months:

Is the ground moist? Do the trees appear to be receiving heavy watering often and water standing around the trees? Is the ground heavy clay? Decrease watering.
Are the trees planted too deep? The tree part between the trunk and the roots is called the plate and it should not be underground. Trees will suffocate ea, especially if overwatered.
Is the ground dry? Do the trees appear to be receiving good watering 3x a week? If not, increase watering.

Trees planted over 12 months ago:

Has there been any chemicals introduced? Remove or stop the chemical stress.

Over fertilizing.
Accidental spraying of weed killer.
Large dog or human urine.
Dumping of other matter which altered the PH more alkaline (Soap, Baking Soda, etc.)
Insects or Fungus? If water is not an issue, this is the most likely suspect. Spraying broad spectrum chemicals such as those sold at most hardware stores for roses should work.
Shade? Does not kill trees, just slows their growth.

Often there is not a single reason, but several reasons weakening the trees.

It is always advisable to spoil your trees with spreading compost, mulching and fertilizing multiple times a year. This optimizes tree growth and makes them less susceptible to die offs due to whatever stress the future beholds.

Beautiful Green Giant Arborvitae Thuja, looks as if nobody showed them TLC for years, but very healthy; Lexington, KY