Women around the world find their tits suddenly exploding to ridiculous proportions.

It doesn't happen to all women at once, however. Starting tomorrow, a small number of women find an odd king of itching pressure building within their chests that quickly turns into a warm burbling as their mammary tissues kick into overdrive with production of hot fresh milk, soaking the tissues inside their breasts to bursting, though the nipples and brain haven't been hit with enough hormones yet to cause to nipples to express milk, so every woman is driven nuts by the pressure building even as their tits magically expand to help compensate.

A woman's tits will quickly expand within 10 minutes to be about as large as her tits could realistically expect to be on her frame, but the milk production within them doesn't stop, even as the nipples remain plugged up.

That pressure continues to build for another 10 minutes, with the woman's already embarrassingly-enlarged tits quivering with pain (mixed with a bizarre kind of itching pleasure deep within that mammary tissue, causing the woman to get horrifically horny even as she weeps in pain and embarrassment at what's happening to the top half of her body), until after the end of that second 10 minutes, at which point all that building pressure will cause the breasts to explode in size one last time, pushing the woman from "ridiculously-but-realistically large-breasted" to "human cartoon hucow" in an instant as her nipples finally release and shoot gallons of hot fresh milk all over the place.

Again, this starts off happening to a relatively small number of women (maybe fewer than a million), but keeps happening to more and more women (seemingly without any pattern) until after a year, every breeding-aged woman on Earth has been turned into a milk-producing hucow.