Answer: There are several reasons why this would happen, including hardiness of the specific ivy variety, the moisture level of the soil all winter, the amount of wind the plant was exposed to, and the effects of heat or sun reflected off the building. In a container, the roots are not as insulated as they would be in the ground and the soil dries out faster than the ground would, so it may be that the plant should have been watered during the winter and/or the container should have been insulated in some way.
Since it seems that the stems are still alive, trim it back somewhat, remove the browned leaves, and keep an eye on the watering this year. You could also take tip cuttings each summer to grow indoors as a houseplant over the winter and then use that plant as insurance against losing the outdoor plant. You could also use an antidessicant or antitranspirant spray on the foliage of the outside plant next winter. Also take care to shelter it from harsh winter winds.
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