I notice that your shrub is in a container; its roots may have grown too large for the pot and therefore, the plant might not be getting enough nutrients or water to all its needles. As a result, bottom needles and sections of the plant die which should help feed or hydrate the rest of the plant. It is also possible that there is a disease that is affecting this shrub.

As you suspect that this shrub may be dying, I recommend that you contact a Certified Arborist who would be able to assess the situation better than I. We suggest Certified Arborists because they must pass a test and must acquire continuing education. You can locate such individuals at http://www.treesaregood.org/findanarborist/findanarborist.aspx