Since we were not aware until today that minorities received more hostility on SO, that means users really don't know other users' gender and ethnicity, so it's not like anyone will call you a sissy girl for speaking up.
It's debatable whether women and minorities actually do receive more hostility on SO, they're more likely to speak up, or SO's management is simply more responsive to their complaints.
But it's pointless to debate things that don't and can't change what we choose to do. Notice that all three of your proposed hypothesis can't change the fact that SO isn't very welcoming and they want to change that.
Rather than ignoring or mocking complaints from white men, we could choose to listen.
I personally don't mind though. In my city many positive quality of life changes were made only after some disabled people complained. I benefit from those changes too, so I don't mind the delay as long as it eventually gets done.
And if, as is often the case, past complaints had little effect until a particularly favored interest group spoke up, it's also an opportunity to listen more effectively to people who may have been ignored.
Our culture encourages women and minorities to speak up. White men are expected to endure abuse in silence.
Consider how much attention complaints from women and minorities receive, and how little attention (and how much hostility) complaints from white men receive (unless those men are individually powerful, of course). People tend to repeat actions that are effective, so women and minorities learn to speak up, white men learn to keep their complaints to themselves.
Edit: I'd like to thank those who downvoted this comment for providing a live demonstration of the phenomenon.