Answer: Leaves on evergreen plants don't live forever. They shed leaves seasonally, it just often goes largely unnoticed. I've noticed a pronounced drop of older leaves on a number of evergreen trees and shrubs this year, probably caused by climatic factors.
Probems which can also cause older leaves to yellow and drop include soggy soil conditions and loss of roots (from fungal decay or physical injury). If these last two don't seem to fit, I'd suspect your plants are just transitioning to a new growing season and will be fine. If it progresses and gets worse, call in a professional arborist to investigate further. Hope this helps!
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