Joss Whedon himself said so. His real reason? Twitter is too addicting, and he wants to be more productive, so he's stepping back for a while. He may or may not join Twitter again at a later date. He said the accusation that "militant feminists" drove him away is "horseshit." Though he did offer some remarks about the backlash he got:
Every breed of feminism is attacking every other breed, and every subsection of liberalism is always busy attacking another subsection of liberalism,
I saw a lot of people say, 'Well, the social justice warriors destroyed one of their own!' It's like, Nope. That didn't happen. I saw someone tweet it's because Feminist Frequency pissed on Avengers 2, which for all I know they may have. But literally the second person to write me to ask if I was OK when I dropped out was [Feminist Frequency founder] Anita [Sarkeesian].
I don't expect any feminist-haters to change their ways over this news, but they should probably stop spiking the football about this topic. It's a non-issue. Peace.