From the die back pattern in the pictures, I think this may be a fungal disease called Pestalotiopsis Blight which occurs periodically on arborvitae.  

I am including a link to a Univ. of Illinois article which explains in more detail the symptoms, life cycle and management for this disease. As with any fungus, you will want to prune out the brown foliage and minimize the conditions that promote the spread of the spores. Avoid overcrowding the plants so there is adequate sunlight and air circulation and only water at the base of the shrub. The pruning can be done now, after the shrub is dormant or in early spring. Fertilizing the shrub in the spring may revitalize it for a new push of growth. You can use a copper based fungicide for prevention but that is usually not warranted. Hope that helps.
https://web.extension.illinois.edu/hortanswers/detailProblem.cfm?PathogenID=246