Always sucks to lose a tree. In this case, I think you worked it too aggressively. For a collected tree, it's a good idea to leave the roots alone for at least two years after collecting. Then you left it with very little foliage to power new root growth. Junipers' strength comes from their foliage. When you need to do heavy work on them, the foliage needs to grow out first, i.e. it should get quite bushy. Finally, leaving it on a pole with single digit temps is not a good idea. The ground provides tremendous temperature buffering and prevents circulation of cold air under the pot. I know, it's easy for me to be an armchair quarterback! But these are my best guesses as to what happened.