It could be a lack of water, or a trace element. Try a balanced feed first. Unfortunately, there is a fungal disease called 'Box blight' which hits this plant. If it's that, the plants don't recover and need to be destroyed.

Box/Buxus going orange brown is a sign of trace element deficiency, so a good feed will turn it green again over time. You need to keep feeding over months but you will see a difference. I have the same problem as my box hedge is on a hill so the top plants go orange brown with the rain washing away all the nutrients and the box at the bottom are a lovely green.

Box blight makes Box look dead - yellow/brown and moldy.