I guess my experience is that it's down to guys wanting to impress the people around them - their mates or women around. I suspect they're insecure or like fighting - most likely both. I'm meaning random picking a fight on a stranger there; just because they're different or an easy target.
I'm betting the ones picking on you have either a pal or group around them or if on they're own, they're a bully who's from the same school/town.
I can't remember the last time one guy on his own ever bothered me to fight me - they nearly always need the security of their pals.
1. I used to have an issue with dudes who would come up and run into me or whatever to make their asshat friends laugh. I ignored it for a while until one day, after one particular fucksack fake-jostled me one too many times, I lost my shit and just punted in the general direction the push came from. By pure luck, I managed to nail the dude directly in the taint. I might have been wearing my steel toes that day. Who is stupid enough to pick on a girl in steel toed boots? Really?
2. Once upon a time, before the wonders of homeschooling, my little sister got the shit beat out of her by some fuckstick on her way home from school.The kid who did it was some obviously ridiculously stupid, but otherwise mentally stable high school girl, trying to impress her friends by beating up an awkward, autistic middle schooler. It wasn't the first time Sarah's been physically threatened, either. Who the fuck picks on an autistic kid? Who the hell does that?
I guess what I'm trying to say is that some people need to compensate for their teeny tiny nads (literal and figurative) by picking on people they perceive as weaklings. Also, wear large boots.
Scream like the punk out of Police Academy II...acting crazy puts folk off-guard. XD Even putting on an evil grin disconcerts folk. But best of all, don't stop for them and give them time to work up the courage to hit you.