Prune out dead branches. Most likely your Leylands have a cultural or environmental problem. Shading will cause these symptoms. Yours are planted close together and have grown closer; consider removing every other tree. Stressors such as drought cause trees to thin out, too, primarily the interior needles. Also, compacted soil, often caused by mowing on wet soil over the trees' roots zone, makes it difficult for roots to get water or oxygen. Leylands are also susceptible to at least three diseases, one of which — Cercospora needle blight — causes browning and dieback of lower, interior branches and moves up the tree. This disease is associated with wet weather or tight foliage that cannot dry quickly after it rains. For a more positive diagnosis, have an arborist certified by the International Society of Arboriculture look at the tree. They usually come out at no charge for an on-site diagnosis. Locate a certified arborist near you at treesaregood.org.