Maybe because they have been through a tragedy in the past and try to take there anger out on something, and it turns out to be a terrible incident that has to do with breaking the laws and governmental stuff.
Answer:
The reason why more African Americans settled in Mississippi Alluvial Plain is that this region is ideal for agricultural purposes.
The rivers coming from the mountains bring sediments and have built deep layers of soil here making it ideal for landforming. Ozark Mountains region, on the other hand, lack plain land as there are more forests and lakes.
Apart from that, Africans were brought in Mississippi Alluvial plain as slaves and they worked on this land for a long time. That's is why they are more concentrated in this region.