A rabbit keeps digging food out of its dish why would it do that?

Short answer, boredom. Or your rabbit may be seeking a favorite grain if you feed a grain in addition to a rabbit pellet. Solution get a feeder that hangs from/through your rabbit's cage (they have a wire bottom which allows dust to filter off the feed) and hang about 6 inches up from the floor. This may not be a total solution but will help considerably.