Thank you for this thread.
As for what she was pissed off at, I can only guess, but because of the Trigger, I would think that she was fed up with being treated like a young female in public. Sometimes, it's treated as an Illness. (Also, doors are designed for 6' tall men. So are Kitchen Cabinets, if you like you can do a search for "Short Girl" memes.) The rest of the time, you're constantly reminded that you're young girls in public. Just from demographics, I can basically guarantee that they were both followed, catcalled, or at the most extreme "Oops" assaulted, at least once, in the past 24 hours.
Because that's how young girls are treated in public. If it's not an illness, then it's an invitation for sexual advances. I generally hold the door for people when their arms are full, or otherwise have some problem with operating a door. (Being young, and female, is not such an infirmary.)
Since you asked, I would have to guess, but most likely she sounded fed up with being treated like a Young Lady In Public. Go out with one, I'm sure you know, or maybe even 2, and just follow them around. (If they're your friends, don't follow strange girls around in pairs.) Just keep your distance, and watch what thy put up with, constantly, as long as they're out in public. They're judged on their weight. There's no perfect Goldilox zone, where they're thin enough to be considered healthy, but not thin enough to be considered vain.
If they dress nice, they're harassed for it. If they don't dress nice, they're harassed for it. If they don't Smile, they're harassed for it. This isn't Just men, women do it too. You want constant fashion advice from complete strangers? Try being a woman in her early 20s. You ever get comments on your Shoes? When was the last time someone told you you wore the wrong shoes today?
It sucks, to be them. I'm sorry she took it out on you, but after that, they ran off, her friend was embarrassed, and then it continued to suck to be them, until they got home. Girls that age have to maintain a standard higher than the Military Uniform Discipline. Basically, and worry about sexual assaults. It's stressfull.
I'm not excusing for what she did, but you asked. I can only offer a plausible explanation.