Autism

If you're not familiar with it, start with the double empathy problem.

The challenge is that we do not have the same social rules and behaviours as the NT population. They are used to specific body language cues and small talk; unless we mask, we don't provide all the right body language (let alone small talk), which impacts how they read us. For example, plenty of people on the spectrum can have a relatively flat affect compared to NTs, leading to all kinds of misunderstandings. We also respond differently to the same sensory input because for many ND individuals the senses are over-reactive or under-reactive.

NTs usually default to viewing the world through their social and behavioural rules, so unless they know you're autistic and what that means, they can often misinterpret autistic body language. Unfortunately ND behaviour is often perceived as arrogant/hurtful (our "brutal honesty"), distant/cold (flat affect), lacking a sense of humour or appropriateness (either not laughing or laughing at the wrong things), etc. which does not incline them to like us.

As for stimming, unfortunately in many regions it is not considered socially acceptable. I've also seen it in (usually older) movies and TV shows associated with characters who are unstable or dangerous. So a random NT seeing a random autistic stimming on public transport or at a mall/shopping centre may assume there's something "wrong" with the ND individual and assess them as a potential threat.