Question
In Spring of 2004 I had 25 Leyland Cypress trees planted in my backyard which faces a main road. There are other established trees in the yard as well.  They did well during the Summer - I watered by hand every couple of days etc.  After the Winter I noticed that most of the branches on the trees were turning brown.  After some research on the web it appears that they may have something called twig & branch dieback.  

Answer
Marie,
There is a possiblitiy that it is branch and twig dieback effecting the cypresses at this time. Fungicides applied at this time would help to control the spread of this disease.
If you live in an area that gets snow fall, the other cause for the condition could be salt burn from the snading and salting of the roads to keep them clear.
In either case, fertilize them well this spring to help them recover and carefully prune out any dead branches.