I suspect it's root rot. It's a fungus-like organism that feeds on unhealthy roots and grows to feed on healthy roots. Likely due to soil being too packed and too wet for too long.
Fix: Amend the soil for aeration/drainage (I prefer diatomaceous earth (#8822 oil absorber from Napa). Let the soil dry to 1/2" down between watering.
More time-consuming but much better fix: Dig it up, remove surrounding soil, prune/treat the roots, and replace.
It's not my favorite website, but:
Other, less-likely possibilities:
Pests. Boxwoods are fairly vulnerable plants. Use an insecticide in case it's Leaf Miners. May be root worms. Either way, feed regularly (not too much).
Too much love. Could be as simple as overwatering / overfeeding. Waxy-leafed plants don't show early signs of TML like others do. Fix: Find a companion plant. (idk the proper one for Boxwoods, but it's similar to planting Impatiens with Rhododendrons, because the Impatiens will let you know the watering situation long before the Rhodos)
Mulch. Make sure there's a 2" gap between the trunk of the Boxwood and the mulch, or this can seriously increase the risk of root rot. Also, if that's pine and fresh, it's possible that dust contributed to a pH spike that the Boxwood didn't like. Fix: make sure it's away from trunk and test the pH of the soil.