Answer: There are several possiblities that come to mind. First off, transplanting is very stressfull to plants. A larger evergreen shrub would ideally be root pruned well in advance of the move, then moved in early spring, and then mulched and watered religiously until re-established. The watering also needs to be monitored all fall and into winter until the ground freezes. So it is posible that the move weakened the plant and the drought many areas experienced last summer and fall weakened it further to the point where it has died.
Another possibility, and this would apply to all of the random plants, is that the shrub was for some reason already stressed and in a somewhat marginal state and so the drought or the winter did it in. Stress can be caused by many factors from lack of water, root competition, poor rooting in general, undiagnosed pest or disease problems, an odd pocket of soil different from the surrounding soil, a change in drainage patterns or other root/soil disturbance, excess road salt or fertilizer, herbicide drift, an isolated accident or incident such as lawnmower or weedwhacker damage to the trunk, and so on.
Finally, some varieties of arborvitae simply change color during the winter and in some people's eyes, look quite dead when in fact they are just fine and will green up again come spring.
If the problem seems to be spreading, you might consult with a professional arborist or with your County Extension (305-5742) for possible causes. While arborvitae for the most part are quite sturdy plants they do occasionally suffer from root rot and other disorders including disease and insect problems which could possibly spread from plant to plant.
I'm sorry about your shrub!
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