leland cypress trees
There are likely multiple things going on. Leyland Cypress was the 'go to' landscape plant for many years for evergreen screening in yards. In recent years it has become evident that they look great for the first 15 years or so, and then start to decline. Very often we see them planted too close together, which leads to competition for water and nutrients. As the trees mature, lack of sunlight leads to the decline of interior branches.
Stressful conditions like drought or cold winters can make Leyland cypresses susceptible to different insect pests and diseases. Common problems include 'winter burn', which is a browning desiccation injury from drying winter winds. We have seen a lot of this type of injury in Leylands this year. Here is a recent article from our plant pathologist about this: https://marylandgrows.umd.edu/2018/04/11/why-is-leyland-cypress-turning-brown-winter-took-its-toll/
Much more serious are canker diseases and needle blight. Here is a page about these diseases and the symptoms to look for: http://extension.umd.edu/hgic/plants/seiridium-and-botryosphaeria-canker-leylands-trees No fungicides are effective for these diseases.
In addition, bagworms are an insect pest of Leyland cypress. If not controlled, these can also lead to browning and decline. https://extension.umd.edu/hgic/bagworms-trees-and-shrubs
For winter damage, there is not much you can do other than wait to see if new growth will develop and then prune out affected areas. Keep the trees well watered and mulched in times of drought. Avoid pruning when the trees are damp.
The bottom line is that Leyland cypresses look good for about 10-15 years and then they start to decline.