White women with big asses have been around forever. What happened was the media (largely controlled by white men) has consistently depicted the look of the ideal woman as white, small waste, small ass, and big tits. And for a long time, the public has tried to achieve this "ideal woman" look, but in reality, most woman can't even come close to that standard.
Then 2 significant things happened which caused big ass women to accept that the way they look is loved by many men, and allowed white men to openly acknowledge their love of big asses. 1) hip hop culture became mainstream. Now, more often than not, you see big ass black woman in music videos "dropping it like it's hot". 2) porn became mainstream. Now, you can see all shapes and sizes of attractive women fucking in videos released from major companies, not just underground stuff.
So ultimately, once the white public's ideals on appearance changed, the white media had to change, if only for the money that could be made. Then the cycle begins anew. Because once the media embraces something, they then begin to influence peoples thinking and habits. It's the nature of the media.