Hello, this is a tough question.  Each different cedar species has distinct issues.  Can you send photos?  We have western red cedar, incense cedar, Port Orford cedar, and Alaska yellow cedar in Oregon, and there are some additional horticultural species.  A picture of the tree and foliage, and some of the damage would help.  Do you know which species of cedar it is?  Port Orford cedar in particular has a root disease (Phytophthora lateralis) that kills trees, often one by one in a line.  

Hello, it looks like you may have both incense cedar and port orford cedar in those pictures.  The heartrot you show in the cut section probably would not lead to tree death, it is known to occur in some cedars and can persist many year, and is not thought to kill trees.  The holes in the bark are a bit confusing for me.  I have not seen that before. Was that the dead tree?  As wood borers are known in dead trees, but not really in live trees. 

The dead foliage may be an indicator of decline, but it may also be fall foliage drop.  In cedars they drop the entire branchlet with leaves. and after a dry year, there can be a lot of foliage/twiglet browning.

If you do have port orford cedar, that is the tree with a non-native disease issue, and it causes tree death (port orford cedar root disease: https://pnwhandbooks.org/plantdisease/host-disease/cedar-port-orford-chamaecyparis-lawsoniana-root-rot)

I apologize however, as I'm not certain what could be killing these trees.  Drought is certainly another issue.  You may want an arborist to visit, but of course that involves costs.