The most plausible theory I've heard is it originated as people meming Henry Hill's coked up rants at his wife Karen from "Goodfellas" and then grew from there.
I though it all started over a year ago when that woman called the cops on black people that were in a park barbecuing because they were black and barbecuing in the park, whose name actually was actually Becky. Then some woman named Patty called the cops some black kid for selling umbrellas on the sidewalk. Those common names just became a generic white woman name like Karen.