GOLDEN LAYLAND CYPRESSS PROBLEM
You did not mention how old the trees are and the site conditions.
We are hearing of many problems and decline of leyland cypress, which begin to show up at the 15-20 year mark. They are not really well suited to our climate long term. Stresses such as winter injury, too much shade, planting too deeply, drought, poor drainage, etc.,
can lead to disease and insect (bagworms) issues.
Last season and this spring we had abnormal rainfall. This reduces oxygen in the soil and roots can be affected. We had a drought this summer and this affects the root system as well.
Here is more information, from our Maryland Grows blog recently written by our plant pathologist:
and the diseases most prevalent: https://extension.umd.edu/hgic/topics/seiridium-and-botryosphaeria-canker-leylands-trees
All you can do is prune out dead branches. Check the drainage in the area, keep watered during dry periods, and make sure mulch is no thicker than several inches and keep away from the base of the trunk. Monitor for additional symptoms.
We purchased the trees 5 years ago. They were about 8 feet tall when planted.. they are in our yard and indeed in the wind. But do not create a wind break. They are separated by about 20 feet. i think your information covered our problem. But if the above information changes the situation, a response will be appreciated. Thank you.
Based on your photos the branch dieback looks to be due to a type of root issue.
At this point, we recommend removing the stakes and any wire to make sure it is not girdling the trunks.
WHO WILL DEAL WITH SUSPECTED ROOT ISSUES?
If the tree is important to you, you could consider having a tree health expert, called a certified arborist (credentialed by the International Society of Arboriculture or ISA) take a look, evaluate and make recommendations. Most larger tree companies have them on staff, or you can search for one at www.treesaregood.org
Given the problems that Leylands chronically and eventually have, it may not be worth the expense.
Otherwise, as noted above, prune out dead material and monitor the tree.
Root issues can be caused by multiple issues, but being planted or mulched too deeply is a common one.
This and others can be explored here: https://extension.umd.edu/hgic/topics/cultural-and-environmental-0