I am watching the video anyway at the moment, because I like to play armchair psychologist for a little bit.
At 2:10 you can basically see her Autism in action. She thinks people don't talk to her directly, when she is with someone, because she thinks they think that she can't hear them.
That's wrong though and it's basically her Autism in action.
The real reason is of course, because you can tell she likely has a mental problem just by looking at her. The headphones just add to that. She thinks mechanically they are just there to alleviate her from background noise and since that are their function, that has to be the reason. Some young people walk around with headphones to listen to music. They don't get treated like this. Normal people naturally think that people with mental deficiences are like children. In need of protection, but also with no or not as much agency. So they naturally speak to the "caretaker", instead of treating her like an independent, fully autonomous person. I guess in her case she always has a caretaker with her for anytime she goes outside.
So the real reason is that people just naturally don't accept her as a fully indendent person with full agency. Just like the guy in the video with her is, because imagine him asking a normal person all this questions. He naturally treats retarded people in his videos like children. Even the ones with physical deformities.
He's not wrong doing that. That is just what people normally do. But I bet many people that are just physically crippled, but still mentally fit would not like to be treated like children.