I too have noticed this phenomenon, over and over, although I hesitate to justify stereotypes. I have asked African-American friends I trust to understand my motivation, and have found no universal answer. My thought is that dogs were used as far back as slavery to keep the slaves at work and in line. Although my neighbors, who are currently petrified of dogs, obviously have no direct memory of slavery and dogs, they were raised by people who were raised by people who were raised by people, etc., who would have been menaced by animals. It seems to me that this fear would be easily passed down through generations who had no direct reason to justify the fear.