It's not possible to diagnose with certainty the cause of dieback from this distance, but likely causes include root damage, root stress from too much or too little soil moisture (last year's drought is a possibility), or either of the two common fungal diseases that can infect leyland cypress. Ideally, dieback (regardless of cause) should be pruned out, but in this situation only an arborist would be likely able to accomplish that. A certified arborist also might be able to determine what is stressing or damaging the tree overall, since they'd be able to see more detail in person than we can. Arborists cannot treat or cure every condition, but can help narrow-down a diagnosis and perform pruning as needed to remove weak or dead wood. If you suspect drought damage, monitor the tree(s) for watering needs by feeling the soil about six inches deep in the root zone. If somewhat dry to the touch at that depth, watering will probably benefit the plants; if damp when checked, watering can probably wait.

Miri