It is difficult to pinpoint one ailment to the Junipers as many factors could cause this dieback. 

It could be a fungal blight, root rot, vole damage, or a broken or damaged stem underneath that you can't see. 

You can prune out the dead branches and see if they can repair and regrow. Junipers can put out new growth but if the dieback is too severe they won't regrow from that section. 

You can look for gnawing marks on the branches which would be indicative of vole chewing. 

Are they irrigated with a sprinkler and is the soil staying too wet? They look like they are in a sunny location but if the soil is poor draining that could cause root issues. 

Regardless of the issue causing dieback, you can replant as any disease would stay with the plant material but if the site conditions aren't favorable, maybe another plant would be a better option. 

We can help with suggestions if needed. 

Emily