There was a 'flip" of the pronunciation sometime in the late 70s/early 80s. Black Americans began pronouncing it as a Brit would say it--'ought' sound. Before then, it was always like the bug--'ant'.
I believe it had to do with wanting to sound more sophisticated, as growing up as a black man or woman in the USA in the 70s and 80s was still troubled, especially living and working in predominately Caucasian areas. Most likely a well known figure like Jesse Jackson or a more controversial one like Don King used it like that in a speech and it caught on.
Personally I think it sounds very ridiculous when you hear that same person saying "axe" for ask and other Ebonic pronunciations, but they hold on fast to the "ought" sound for aunt.

I grew up in Texas. Everyone and their dog, regardless of race, says it like the insect--'ant'. American blacks regardless of where they are from within the U.S. all now say it like 'ought' sound, because sometime in the 80s, some big shot black celeb/politician (most likely Jesse Jackson) decided to say it that way in a speech. Blacks all over the country picked up on it, so as to sound "more sophisticated". I can promise you up until that time, blacks in the South all said it 'ANT', as did N. E. Coast blacks. It's sounds so ridiculous, because when you got all these blacks saying things like 'axe' for ask and 'wutchyou...' for 'what are you...', but got to say all poshy Brit-like 'aunt', it's like Eliza Doolittle posturing.