I actually have a similar question about my Blue atlas cedar. Purchased in April, chopped the trunk and it recovered nicely. Then, suddenly, a month ago the needles started dropping one branch at a time. I live in Atlanta, GA. Its in a well-draining pumice mix. I watered it regularly since it dries up pretty fast in the summer and slowed down as the temps have started to drop and the soil stays wet longer. Not sure what's causing this, I'm positive the bottom of the tree is alive and green but the top third feels dry and dead.