I know exactly what you mean. Very very chilling when i think about it.
This is NOT scientific, but this is what I was told by a bouncer when I started tending bar. I'm not putting it in quotes because I don't remember the exact words from 20 years ago:
It's not enough to be alert and prepared, those assholes have to see that you look alert and prepared. They look for soft easy prey. If you look like you're ready for trouble, they'll look for a woman who isn't. That's a cold-ass thing to say, but they really do think that way. And yeah, you'll hear some shit about guys looking for tough chicks to teach them a lesson. Thing is, guys who go for subduing tough chicks who know how to fight get fucked up sooner and don't last long. That's why they stand out.
Again, not scientific at all, but it is the very well informed opinion of a man who as an experienced bouncer had seen the seamier side of life for a long long time.