This is a great natural behaviour and perfectly normal. It shows us exactly how valued food items (and toys) are and how important ownership of such items are to our dogs. We ignore these very strong messages at our peril!
Dogs are domesticated wolves and are driven by the same instincts, needs and social boundaries. Food items that lower members of the pack are allowed are the ones that the alpha animals do not want or need, this in itself shows the dog that the pack is working well and the leaders are strong enough to provide for the pack. Happy dog. Problems can arise when the leaders start to mess around with this very simple concept - if you give me food, it is mine, you have no right to change your mind or ask for it back, by giving me food you are telling me that there is more than enough to satisfy you and also to feed me.
Some dogs retain the instinct to cache what food they are allowed that is not critical to their survival - in wolves this serves the life saving purpose of ensuring a food supply for an individual when hunting success is low or there are changes in pack structure. For the early domestic dog it was a useful trait to retain as the flow of scraps and leftovers from the master's table was not a reliable one.
It is simply a instinctual behaviour that is fascinating to watch and tells a lot about the way the dog has evolved.