You don't say where you live, but one guess is that it might be cold damage. Boxwoods can be very tender, and frost damage on them can be quite random. If they recover and are looking healthy (except for being somewhat misshapen by the dieback), it's probably winter damage.
If they are in an exposed site, this winter consider covering them with burlap for extra protection. Another possibility is that your boxwood are suffering from something called boxwood decline. Plants are weak and spindly and the dead branches are rather random. Old foliage yellows and drops. Leaves may have pink and black marks and cankers may form near at the base of the plant. There's not a good "cure" for boxwood decline. However, nematodes may contribute to the problem, so you can have your local extension office check for nematodes. Otherwise, if your plants are English boxwood, you may want to reconsider replanting with American boxwood, which is more resistant.