Biologically, some people naturally have enlarged amygdala's (the fear centers in our brains), and others, due to several reasons will develop enlarged amygdala's due to traumatic events, irrational thoughts, cognitive difficulties, how they take in information, their physical and social environments, etc. These areas of the brain were most useful when we were "3 hairs from being a baboon" living amongst apex predators in the wild. Our brains had to be on "high alert" all the time. However, now-a-days, it tends to be more of an impediment. Fear and anxiety tends to override rational thinking, and in some cases can be debilitating enough to severely affect day-to-day living. In milder cases, it can lead to irrational thinking, not understanding context and perspective, creating and/or believing conspiracy theories, or even something as simple as having a "negative" outlook or not applying for that promotion at work. It's a difficult thing to overcome.
Our media and political leaders have learned to manipulate thoughts and behaviors through fear, as well. There's actually been a handful of studies to strongly suggest that those that have anxieties regarding change and social situations are more likely to be "conservative". So, it's no wonder that conservative political adds in the US are specifically designed to trigger anxiety and fear.
We can sit here behind a computer screen and talk about rational thinking strategies and day-to-day risk analysis as a means of overcoming something like agoraphobia, PTSD, OCD, or anything else that has an underlying fear and anxiety component. However, that's no easy task and in some cases, can take years of therapy to overcome.
As far as fear and anxieties specifically within the autistic population, the science is pretty clear that, "statistically speaking", we are more prone to this phenomenon. Brain imaging studies of autistics vs. neurotypical controls also suggest statistically larger amygdala's in autistics, especially in children, and less so with adults. Some studies suggesting that with time and development, autistics begin to rely more upon their rational thinking centers.