Answer: Your landscaper was probably thinking of an antidessicant or antitranspirant spray which coats the leaves to limit transpiration or water loss through them. It is often sold for use on broadleaf evergreens.
Junipers are generally very tough, very wind and cold tolerant (Witchita Blue is rated hardy into zone 3) needled evergreens and so it should not be necessary to treat them with anything special for winter survival, especially shrubs that have been in place for three years and are well established.
Junipers are most sensitive to excess water around their roots, especially in the winter time. If something happened to cause flooding or standing water or an excessively moist area around their roots, they would probably die. This seems to be the most frequent cause of juniper death. Maybe this happened to your shrubs.
While I can't tell you exactly what happened to your shrubs, some additional reasons for them to die might have been root or trunk damage, a change in drainage patterns, accidental contact with herbicide, or root rot or some other pest problem. I hope this helps you trouble shoot.
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