Some possible reasons are: Box blight, tom cats or small dogs. Is there a tell tale smell?

It does look like the living branches are putting out vigorous new growth, so I think the plant as a whole seems healthy.

since it is a recently planted boxwood, could be root rot. another possible is animal urine as others have posted. You could cut off all the dead stuff and adopt wait and see stance. sometimes that is enough. however, if it continues to decline, transplant might be required. Also examine wood close to the base of shrub. if you see any holes, that could also be problem and it is usually fatal.