Here is a link to a previous askhistorians thread on the topic. The general consensus seems to be that Chicken became associated with african americans due to the availability of chicken and watermelon because of longstanding stereotypes of african american laziness. Watermelon was also "easily accessible" a way; easy to prepare, easy to eat and apparently easy to grow which is why it was a natural choice for racists wishing to depict african americans in a way that perpetuates the stereotype of laziness.