This saves the remaining animals the hardship of competition for resources, and prevents a lengthy starvation by those that would not survive. Feeding animals will prevent many overwinter deaths. The next spring there is a larger than normal population of the animals, increasing competition by an amount commensurate with the number of animals increased. The extra animals cause a greater use of the supporting land. Heavier land use can cause a degradation of the quality of the land.
Most people are familiar with the concept of overgrazing cattle.
Placing too many cattle in a small space destroys the grass the cows use for food, can muddy water the cattle are using to drink, and render that land space useless for raising cattle. The same concept applies when dealing with wildlife. Now let's consider the second winter after feeding. There is now a larger population with a greater proportion that will die over the winter without the aid of feeding. A larger amount of food is needed to support this population, not only because the population is larger, but also because the land is less capable of supporting them.