I honestly think its because some black people are aware of the worldwide stereotypes of blacks (loud, irresponsible, violent, lazy, unintelligent, unproffesional, etc). Then on top of that we are the minority in most places so some black people feel a pressure to go AGAINST the mainstream stereotypes of black people in order to be taken seriously.
Sadly tho, merely acting "professional" and/or talking and dressing a certain way is considered "acting white" or looked at as trying to appease to whites to a segment of black people, even if that's not the intention. This all goes back to how we view ourselves and each other. Some black people truly think that if you don't fit a certain stereotype, you're "less than black".