My son has been recently diagnosed with ASD at age 25 mo. He constantly rubs his eyes. We thought it could be allergies, never thought of it as a stim behavior. Interesting question, anxious to hear some feedback.
It could be it starts off as a reaction to itchy eyes but becomes more like a stem if it's continued. I used to rub my eyes a lot, because it felt better. Also it removed dried "sandman" stuff at the corners of my eyes. I do have allergies and my eyes itch from time to time, like right now. (thanks to the maple trees outside and spring). I don't rub my eyes unless I have washed my hands or use a clean tissue. As a kid though it didn't bother me.
Not that this behavior is strictly related to autism, but hypersensitivity can lead to these behaviors and Hypersensitivity is pretty common with autism.