Browning on newly planted Leyland cypress could be caused by a number of different things. Perhaps the two trees that are turning brown are suffering from moisture stress. If their roots haven't spread into surrounding soil, newly planted trees need close attention to watering. Essentially, the roots are contained in a small ball, just like in a pot, and they may need watering daily to prevent water stress.
It's also possible that insects, such as spider mites could be attacking these plants. Check them closely for any sign of webbing or insect feeding. Some diseases, too, can cause browning of foliage. Without actually examining the plants for signs of fungus, they can't be accurately diagnosed. You could take a sample of the tissue at the margin of the brown area (include some healthy tissue as well as dead) to the nursery, or to your local cooperative extension service, for examination to see if they can identify any specific pest associated with the browning.