I understand how you could miss it, but the sequiter is "unexpected behavior".

Why do mentally ill people cause some people anxiety? Because they may behave erratically.* In other words, we have a lot more difficulty ascertaining what they are thinking, and by extension, predicting their behavior. The same can be said for the general public sentiment towards AVs.

* I also understand "mental illness" is a blanket category and I'm not using it as a stigmatizing term. It may be stretching the analogy too much, but it's simply a proxy for "I have a hard time predicting what's going through this persons mind"