I only had people yelling from their cars at me some in my teens, but it doesn't sound quite as bad. (And nowadays I don't stim very much.)
People seem to have a really sensitive 'radar' for tiny differences in how people move and look. And stimming that doesn't look "normal enough" seems to be really noticeable to most people.
I don't know exactly why people stare/look-at-person-weird, just that they tend to do that when they pick up a difference of that sort. It's rude to stare/look-weird at people, though, so they shouldn't be being jerks (scaring people) like that.
It's not really an issue for me these days, but there's probably (hopefully) a few people around here who also have or have had to deal with that.