The short answer is that nobody really knows, although it probably has some genetic component. Wikipedia has a long article covering the different hypotheses: https://en.wikipedia.org/wiki/Causes_of_autism

In psychology how we define a thing is constructed from a series of observable characteristics.

The problem is, there are multiple situations that can cause the same series of characteristics. "Are there leading candidate hypotheses for the cause of autism?" is like asking, "Are there leading candidate hypotheses for the cause of a headache?" There is no single cause.

What we do know is most people with autism do not process facial expressions as much as the average person. This causes a lack of paying attention to body language, which then ripples into most of the characteristics that define autism. Studies have shown eye training on infants can completely remove symptoms of autism.

But a root cause is more complex when there are multiple. Some people get overloaded when they look at people's faces. Some just didn't pick up the proper habits of looking at others when interacting with them. The root cause can vary from person to person.

There are studies that shows that all of those things have effects on the likelyhood of a person becoming autistic.

Autism is a complex mental thought pattern that I think is best described as having “erroneous“ metacoginitive models. That is people with autism has problems “comprehending” how the mind works on a intuitive level. Not “comprehending” the mind then leads to them having a hard time following other people’s thought trains. And various other problems.

Why people get in to that thought pattern differs between individuals. So “Ultimate” cause is not so easy.

because the definition consists of many different phenomena and in places does not specify whether all or some should be present, i suspect there is never going to be any single cause.