|Wild relatives of the dog often bury their food. They may have killed an animal that is too big to eat all at once, or may have killed several animals when they were very plentiful. Wolves and fox will bury this food to preserve it and hopefully to protect it from being found by another animal who would want to eat it. This is called 'caching' or 'hoarding' their food. Squirrels do the same thing.
Domestic dogs still carry this hoarding trait and will bury a bone or toy in the yard, or even under a blanket as a way of saving and protecting it. Our dogs seldom experience the extreme hunger that would encourage them to find it again, so often, these buried treasures are forgotten.