Humans are the top "predator" of a cow. Other animals that can kill cattle are cougars, black bears, grizzly bears, lions, tigers, wolves, bacteria, viruses, etc.
Mainly because the animal is either unadoptable, for example, an illness/injury is too severe or expensive to cure, or the animal is aggressive towards other animals or people. However decent animals...
sadly to say they don't have the money or space to keep every animal and if it is not found a home for within a certain time period (usually 7-10 days) it will havbe to be put to sleep to make room...