So the way to look more sexy was to start looking weird? That sounds rather arbitrary to me. Why not grow three boobs or develop wings or whatever? Anything can be explained like that.
I've heard it argued that the existence of decorative traits indicates fitness because it shows that an organism has a enough extra energy to devote to decoration.
But that's speculative. Sexual selection isn't really a logical proces. It's just an accumulating response to stimuli, over time.
In this theory, for some reason the women with those traits (which in all likelyhood existed before) became to be seen more desirable, thus lead to more offspring, thus becoming dominant.