I mostly reject the "It's Feminism!" argument because it's not particularly empirical. I do, however, love to consider such issues from an evolutionary vantage point. After all, we are, first and foremost, animals largely driven by primitive animal instincts with rationality being a relatively new invention. I would venture to guess the answer lies in technology and the stabilization of society.
No longer are we threatened by lions and no longer must we come face-to-face with zebras to alleviate our daily anxiety of where our next meal will come. When you see people pray before a meal, remember the ritual originated from literally having to be thankful to some imaginary god for allowing you to have food. Lifespans have astronomically soared thanks to the marvels of modern medicine. Also, for the most part, we also no longer face wars, foreign invasions, and the Klu Kux Klan. Instead, now, our society freaks out when a serial killer murders a handful of people out of milions, over years, which is horrible but nothing compared to threats of the past. Turn on the television news and there's amost always some new scare-of-the-week. I wouldn't be suprised if next week it's "dangerous" to sit in a chair.
With most threats sanitized, naturally society would then turn towards postmodern philosophy and systematically question the validity of gender roles, both biologically and interpersonally/socially. 'Why are guys be such asswholes?" "Why can't women have career and family life?" Naturally, androgyny arises.