Bronze foliage is one of the most commonly identified "afflictions" of yew generally and can be especially noticeable yew hedging when individual plants can go bronze and contrast with their neighbours
There are a number of reasons as to why yew hedge plants go bronze. With one exception all of them signify that the plant is stressed in some way.
The list of reasons includes:
New Growth. This is the exception - young soft yew growth is very often a light bronze in colour and it greens up as it hardens off.
For all the other reasons, as in "calories in, calories out" you are trying to find and fix the in/out equation. Where it is not because of new growth, bronzing always an imbalance...
Windburn. The leaves on a yew hedge can dessicate in cold winds. Leaf colour can also change with other climatic extremes - heavy frosts, hot summers and so on. The key is a sudden as opposed to a gradual change in the weather. Leave them alone and they will regain their colour. The roots cannot pump enough water to the foliage to keep it green. Too much water out, not enough in.
Root rot. Real care needs to be taken when planting a yew hedge in clay soils or where drainage is bad. While the plants are establishing, they are susceptible to phytopthera infection if the soil remains very wet for extended periods. If the plants have root rot, the bronzing tends to start where the leaf is attached to the stem. The key usually is that planting was in a trench and that the worst affected plants are at the lowest part (so water from the rest of the trench flows down to them). If you can improve the drainage do so, otherwise, leave the plants alone, cross your fingers and hope. If they die, do not replant in the same place because history repeats itself. Too much water in, not enough out.
Starvation. With bare root English yew this is relatively unusual. It is common however when container grown yew is planted out. It happens either because the plants were potbound, or because the ground into which they were planted was prepared badly (not improved properly or the planting holes were too small) and roots are struggling to establish in the surrounding soil. In either case, gently dig up a plant - if the rootball is still pot-shaped you know the problem. Tease out the roots, enlarge the hole, mix in a bit of compost with the surrounding soil and replant. Not enough food in....
Passers by. Cats which like to bury you know what, and dogs who like to let their mates know where they have been are capable of adding unwanted organic matter and relatively high levels of uric acid in a concentrated area. Stop them and your yew hedge will recover. Too much uncomposted wate material in....
Temperament. Sometimes it just does. Theories abound, but in our (considerable) experience yew can be a bit temperamental after transplanting. Put it down to shock.
In general bronzing passes. Yew hedge plants are enormously tough and recover even when they look dead and gone. Always leave them alone until you are sure they are dead and even then dig one up first to make sure there is no white root showing (always evidence of life). In general, if the bronzing begins at the tips of the leaves, it is probably not serious.
Bronzing is not terrifying - it is unlikely to happen at all and if it does your plants will almost certainly recover quickly. Patience is the watchword here.