CCD is believed to be caused by several factors.
Pesticide use
Nosema which is believed to be trigged by stress on the bees and moving bees to pollinate crops stresses them
Lack of a variety of food sources. Like us, bees get their nutrition from several different types of plants and if they don't have access to some plants they may be lacking in vital nutrients.
Environment is also being looked at.
CCD is affecting commercial beekeepers the most, which is understandable when you consider those hives are stressed from being moved, the fields they are taken too probably had pesticides used, and lack of other food sources in the area they are taken to. Backyard beekeepers aren't reporting it as often.
Sometimes a colony will abandon a hive because of other factors like not enough food in the area, too many incidences of pests bothering them or too many instances of disease occurring in their area.