I need to know what the exact species is, before making any determination. Common types in our area include western redcedar, Port-Orford-cedar, and incense cedar. Can you reply with some photos (whole tree, plus closeup of any seed cones and a branchlet), or try to identify the species yourself? Here is a link to some photos:
http://oregonstate.edu/trees/conifer_genera/spp/arborvitae_spp.html
http://oregonstate.edu/trees/conifer_genera/spp/incense_cedar_spp.html
http://oregonstate.edu/trees/conifer_genera/spp/white_cedar_spp.html

Some species of false cedars (which all of these are) are susceptible to some common diseases, while others are not, so knowing the species is necessary.

Thanks for using Ask an Expert.

Thanks for your response. It looks like 2 of the trees have cones and 3 don't.

Thanks for the pictures. These are incense cedars. There is a disease called incense cedar branch canker that seemed to intensify last year. That may be what is going on. Look for sunken, discolored areas along the affected branches. Here is a short article that describes the symptoms:
http://blogs.oregonstate.edu/treetopics/2014/06/27/whats-causing-die-back-incense-cedar/
and more here:
http://pnwhandbooks.org/plantdisease/cedar-incense-branch-canker

The only way that we could definitively diagnose this would be to have a sample tested by our Plant Clinic (for a fee), but it seems a likely suspect. I have never seen this kill trees outright, it just makes them look bad. Pruning out the lower dead branches and disposing of them will help to reduce the spread. The drought we are experiencing may also be predisposing the trees to disease, so you can hope for more rain this winter and then water the trees deeply as needed next summer.

Hope this helps.

Amy