Answer: There are several possible explanations for what is happening to your hedge, possibly working in combination. Last year's drought was very stressful for plants, and you may be seeing the results of that. Hedges consist of plants grown in close competition, so any form of stress can be accentuated -- lack of light, competing tree roots, exhausted soil, drought, insect or disease and so on can all contribute to a decline
It is possible that there is some root rot occurring due to poor drainage, but I would have expected that problem to have become apparent before now, sometime over the twelve years, unless there has recently been a change in the area grading that would cause excess water to collect along the hedge.
There is the possibility that voles (or other rodents) have been at work and caused some root and/or stem damage -- their chewing can cause plants to literally fall over, seemingly out of the blue, much as you have described.
Finally, it is true that older hedges do occasionally need to be renewed by cutting them to the ground in very early spring and allowing them to regrow from the roots. While they regrow, they should be pruned as they were in the beginning, meaning cut back every twelve inches or so to force dense branching close to the ground. The overall shape should also be configured so that the top is narrower than the base. This allows sunlight to reach the lower branches and helps keep them leafy.
Although the hedge make look bad now, it should regrow and fill in over time as neighboring plants take up the slack. Alternatively you can start replacements by rotting tip cuttings from healthy sections. This will ensure an exact match of shrub.
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