Very often, the reason they turn brown is not enough water. Be sure to give them plenty.
This is very common with Leyland Cypress trees a somewhat short lived -not recommended evergreen.
If this is not a weather related result possibly critters like insects mites and scale can cause troubles. It might be best to have someone have a look.
For the best local professional advice I would highly suggest contacting your cooperative extension. These offices are manned by volunteer master gardeners on site there waiting to answer the communities questions that know your local situation quite well. If they cannot id the problem a plant specimen can be sent to the local university for diagnosis.
Master gardeners are required to volunteer back designated hours ( plus continuing education) each year to maintain MG certification -this community outreach and education is their goal.
When ever greens turn brown they are dying. Ours nearly winter killed. We trimmed off the brown and kept watering it until it started to grow again. If you catch it with green still on it, they usually need more water if you live in a dry climate.