Answer: It sounds like root rot, but you won't be able to tell without digging the plant and inspecting the root system. Check for mushy or foul-smelling roots, or those that are rusty or brown in color (instead of creamy white inside). If the root system appears okay, then cut through the trunk of the juniper to look for discolored tissue. If the tissue is the same color throughout, you've ruled out diseases. Aside from the above, not much will kill a juniper in the way you describe.
Best wishes with your landscape!
Q&A Library Searching Tips