The Body Spray: A can of body spray is found by someone, and wherever it is sprayed becomes a shapely female behind. The spray will work anywhere on anyone, and never runs out.
Assorbtion: For reasons unknown, women become able to absorb people in their butts, taking some of their traits(even their genitals) and adding to the mass of their butt. The absorbed people are still alive inside the absorber, but they may or may not have any control and may very well become a body part.
Parasitic butt: an outbreak from a secret lab leads to a small city being terrorized by feminine butt like creatures that attach and change the host, living off and becoming a part of their body. They can also replace a previous body parts, or attach somewhere else.
Globalized change: Whether through a disease, a spell, or any other means, the entire planet is turned into feminine lower halves. All people are now just a butt, legs, and their genitalia. This story deals more with the aftermath rather than the transformation, but can have more added. Naturally, there should be some shemale or futanari halves for reproduction.