Thinning of arbovitae - especially on the lower half - is typically due to lack of sunlight to those areas of the trees. If they are being shaded, the plant is unable to grow new foliage. People also tend to prune these plants like eggs - trimming closer, more narrow at the bottom. This can cause the broader, middle and upper branches to shade the lower branches from getting enough sun. Prune arborvitae like a bullet - wider at the bottom, and rounding up to a point at the top. This will allow branches at the bottom to receive the maximum amount of light.