This appears to be normal seasonal leaf shed on arborvitae (Thuja). Although we call groups of plants "evergreen," they actually do shed old leaves, just gradually and at a slower pace than deciduous trees and shrubs. Instead of all foliage at once, it's only their oldest foliage/needles that yellow, brown, and shed, often in autumn. The older foliage will be inner growth, and will especially include needles that are shaded by dense, younger outer growth. The brown debris after they're done shedding (it can take awhile to actually fall off the branches) can be left alone as it won't harm the plant; if an eyesore, you can gently comb it out with your fingers and dispose of it (or compost it or use it as a mulch).

If you're curious, here's a timely article (published today!) by Ohio State discussing this very phenomenon, though its focus is on pines since their autumn needle drop is quite prominent: https://bygl.osu.edu/index.php/node/1886

Monitor evergreens in general for watering needs since we've had periods of warm, dry weather this autumn and rain has been patchy or not very long-lasting. Feel the soil around five inches deep (not including mulch) and, if fairly dry to the touch, the plants would benefit from a good soaking of the root zone. (If moist to the touch at that depth, they should be fine on their own.) Drought conditions in autumn can have negative consequences for trees and shrubs - especially evergreen ones - which sometimes won't become apparent until months later, when active growth resumes.

Miri