The center of black political power or it's justification for power lies in a poor, Democratic base. Any black person who rises above or walks away from that is a threat (consciously or unconsciously) to that power or to that groups' collective self-esteem. They must be discredited and their opinions dismissed thus they are labeled Toms, sell-outs, house negros, whatever works.
A somewhat comparable analogy would be union workers thinking those who come in to fill a job while they are on strike are despicable scabs. Some black kids are harassed by others when they take their schooling seriously. They get labeled as sell-outs for some of the same reasons above.