Cold damage is possible, check for crown gall on these plants. Mine were looking pretty ratty before removal and galls were present. Mildew was a bigger problem, I don't see that on yours.

If water is coming off the surface behind plant is liable to be too wet. This species likes a hot, sunny location in this climate, and as so often the case with so many kinds of plants needs good drainage. There is a clipped hedge near me of 'Silver King' that looks good, but it is in a somewhat enclosed location, open to the sun for many hours and probably pretty hot in summer. Many other shabby specimens, some looking as bad or worse than yours are to be seen around here now. I was just looking at a bunch of sad ones in a local garden center, inside a partly enclosed wintering structure - where, judging from the appearance of other shrubs the cold was not effectively excluded.

And there could have been problems with the euonymus plants due to damp, still air being around them for long periods.

Various kinds of plants going south after cooler, damper weather comes on in fall can be Pseudomonas infestation.

Here is a link that might be useful: Information on Euonymus -- Bacterial Blight, An Online Guide to Plant Disease Control, Oregon State University

That last photo looks like the plant has quite a scale infestation. Might just be not seeing it properly, but that would certainly cause leaf drop if so. I don't see scale on the other photos. The sudden cold caught a lot of plants by surprise and froze leaves, which would explain leaf drop, but do check that last plant for scale on the twigs.

Most other more recently planted 'Silver King' seen here - and there are lots of them - are having similar issues now. Maybe not as much shedding, but definitely with some dead twigs etc. The basic cause was surely the weather last month.

It looks like it's too wet. I'd move it to a protected, sunny site, with good drainage. Don't forget to mulch.
You should have complete control of any water draining off hard, or semi-hard, surfaces. It appears the water draining off the driveway just seeps into the garden wherever it may.
Mike

Root rot is a big deal here and in other parts of the county, especially where conditions are often hot and wet at the same time. Production facilities must be successfully maintained in climates that often have these conditions or losses may be quite heavy. Individual infested plants appear to be common both in local plantings and in sales yards. Water mold "fungi" are related to brown algae and live in water, move onto exposed plants and infest them whenever the right combination of circumstances occurs.

The small 'Green Spire' (different cultivar but same species) I had going on Camano Island looks terrible now, the whole outside of the foliage canopy burnt looking. A planting of larger ones I drive by also looks shabby. Makes me think this selection may not be so hot for our area.