I would have thought the two most likely reasons would be honey fungus or waterlogging.
Honey fungus can spread through the soil by black 'bootlace' structures called rhizomorphs. It will spread from one plant to the next in a hedge so they die off one by one in turn. Look out for clumps of toadstools, the smell of mushrooms and a white fungal growth under the bark of infected plants at the base of the stem. If it is this, the only option is to put in a physical barrier to stop the rhizomorphs spreading such as rubber butyl pond lining buried in the soil to about 18 inches deep, or re-plant with something more tolerant to honey fungus such as Beech or perhaps Photinia. Unfortunately, honey fungus attacks many plant species. The Royal Horticultural Society has good info on this
http://apps.rhs.org.uk/advicesearch/profile.aspx?pid=180
If the soil has become very wet it could be the roots rotting in waterlogged soil. As a result, the plant get enough water to the leaves so you get yellowing of the leaves and them falling off. Do you know how wet the soil is?
Dr Roger Eavis
Evergreenhedging.co.uk