One of the dominant theories about foxes is that they're a result of an attempt to recreate the legendary Inari Ookami creatures. Well... first of all, japs didn't have any idea what should an inari look like. All they've got is a bunch of folklore and religious teachings which, as we know, are not very informative in that regard.
Point is, they screwed up. Nobody can say how close is the resulting creature to the Inari Ookami as it should've been, but the result is those fox-girls we know. Rumors go that the Japs have used actual fox genome in the creation of those creatures, which results in increasingly instinctive behavior. And what a typical carnivorous animal's instincts are like? That's right, seizing territory, hunting, contests of might and, of course, procreation.
At this point, the entirety of the population of Japan, China, both Koreas, and about two-thirds of those Asian countries nobody knows much about, are fox-girls. Eastern Russia is foxed as well. The lack of men in fox-controlled territories (the amount of surviving men is less than ten there) halts their expansion, but if they aren't gonna be stopped, it's only a matter of time before reach Europe and, eventually, America.