Lack of proper socialization. From young age dog needs to be exposed to any possible situation you may encounter down the line. If one does not have black friends that come to the house, it would be advisable to go to parks, stores etc. where the dog can experience the entire spectrum of the population. Of course, a lot of people encourage racist behavior in their dogs and it does not seem to take much for dogs to pick up on this. They have lived with humans for 10s of thousands of years and an instinct to protect from the unfamiliar looking neighboring tribe may have been a real plus in the past. Nowadays it will just get you sued and the dog put down.
Socialization is important in other respects. Don't have kids? Take the dog to the playground. Live in a quiet home - make lots of noise at feeding time, otherwise dog will go nuts every 4th of July when the fireworks start ...
Cheers,
George