Could be root rot.

Use a magnifying glass to check the underside of the leaves. Boxwoods are frequently infected with mites of various kinds. Their sucking the juices from the leaf undersides can cause the leaves to take on a brown or bronze coloration. If this is true of yours, you'll see the webs if the pest is spider mite and the nymphs of other species. Forceful spraying the underside of the leaves with water is very helpful. Summer weight oil and Neem are other low impact remedies. Dispose of any leaves that fall and take preventative measures next year - always easier to prevent than cure. If there's no evidence of insects - there are other possibilities in addition to mites - check with your County Extension Sevice.