Racist German shepherd....Um,,,,any help would be appreciated?

Please don't find offense to this question..... I have a german shepherd dog, and on several occasion's (every time she has seen one) has gotten in what i can only call "WANT TO KILL MODE" when ever she sees an african american. Is this a common thing? is there a reason? if it happened once i would of brushed it away seeing it as merely a bad vibe.. but its every single time she sees such an individual. I am 6' 4" and 260lbs and when she saw the people today she was pulling so hard to get at them it almost pulled me over. This is a dog that usually runs up to strangers and lays at there feet in anticipation to get a belly rub. Im at a point of such confusion i don't really know what to say....and how to say it with out sounding ignorant. Any legitimate answers would be appreciated. Thanks