There are two cankers which affect Leyland cypress. One, the needles fall off when touched, the other, they stay on. Either way, the result is a dying tree. Other factors which affect the trees are drought and waterlogging. If, as you say, it's an established hedge, then waterlooging and rought can probably be discounted, and I rather think yours have been attacked by canker (you can Google it), and they're on their way out. Phil J