Answer: There are a number of reasons why this might happen. Probably the most common is that they are drying out during the fall and then become dessicated over the winter. For newly planted evergreens, the soil needs to be kept slightly moist, but not sopping wet up until it freezes.
If your ground is in fact overly moist, the plants may be suffering from that poor drainage; drainage problems are often worst in spring with snow melt and seasonal rainfall combining to create the wet soil conditions. This could cause them to "drown" and die in spring.
Another possibility, if you are planting live Christmas trees, is that they were not handled properly. For instance, allowing the rootball to dry out or holding them indoors for more than just a few days causes them to become out of sync with the seasons and can cause failure due to temperature shock.
You might want to consult with your local county extension as to why you are having repeated problems and for suggestions as to what to try next. I hope this helps you trouble shoot.
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