Shrubs and even trees develop bare branches or dead leaves in the interior of the plant from lack of sunlight. Your boxwood is probably dense and the sun can't get to those branches

Prune out all the dead and dying branches and open the interior up a little. If that doesn't work you can rejuvinate shrubs but using the 1/3 method. Prune 1/3 of the plant every year for 3 years.