It’s tough to give an initial diagnosis based on this. Root rot is more likely if drainage is poor. Then again, the plants need adequate watering even when mature.
The plants might also be overcrowded. Am I seeing Leyland cypress trees at each end of your arborvitae? Those huge conifers will offer root competition for your hedge, and sunlight too.
How to decide your next step? If you remove the rootball of a partly dead tree, you may be able to see and smell if there is root rot or girdling roots. The fully dead trees I think I see in the photo won’t offer you much information if dug up.
You can’t treat the soil to kill rot, if these have root rot. Sometimes better drainage can be provided, or more careful summer irrigation, to save the remaining trees.
If these are high value plants in your landscape, consider hiring a certified arborist, as making a site visit is very helpful and not a service we offer. A arborist should be able to discuss choices about the bigger trees crowding arborvitae and any danger they may represent to your home.