It's possible this is due, at least in part, to a twig blight; at least two species of fungi tend to be responsible for juniper dieback in our region, and they might even overlap on the same plant if conditions favored infection. (Often, fungal spores can invade plant tissue more effectively when the foliage is wet from rain, dew, or irrigation.) These symptoms can also be caused by stress from environmental conditions, such as overly-wet soil causing root loss and corresponding foliage death or yellowing. This shrub's foliage does look less richly gray-blue in color compared to how 'Blue Star' normally looks, which implies something other than just a foliage fungal outbreak is at work.

Treatment with a fungicide won't offer any benefit at this point, but you can remove as much browned growth as possible, though this might involve sacrificing some of the stems with still-living tips by clipping them off entirely. Combing through the foliage with a gloved hand might be enough to collect brown leaves, since this type of juniper usually sheds dead foliage pretty easily. We aren't certain the plant will regrow from a drastic trimming (many evergreens can't regrow when the cut wood is too old), but it's worth experimenting since the plant might not aesthetically recover from any other clean-up technique for some time either.

If you suspect overwatering, make sure no roof downspout outlets are nearby (or redirect them if possible), plus that the planting isn't watered until the soil around five inches deep is getting relatively dry to the touch.

Miri