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.
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.