Blue arrow juniper hollowing in the middle
There are many potential causes for your juniper's thinning including an insect pest or a disease, drought stress from the very dry summer we experienced last year, or a combination of a cultural problem - related to the plant's growing conditions - and an insect or disease, since plants that are stressed from a cultural problem are more susceptible to diseases and pests.
A soil test, which can be performed at your county cooperative extension office, will show whether the problem is a nutrient deficiency in the soil that can be corrected. You can find more information about the services provided by the Nassau County Cooperative Extension office here:
The following link will take you to a very informative page on the potential pest and diseases of junipers, and how to recognize and treat them:
Another home owner posted a similar question to this site, and you might find the response enlightening, since he described a problem similar to yours.
Consider having a certified arborist come and assess the tree in your yard. He/she will be able to give the tree and the conditions of the site in which it is growing a detailed examination and will suggest any necessary treatments. Most arborists do not charge for such consultations, and will only invoice you for services they provide following the initial consultation.