I'll give it a go, but I'm not an expert. Autism is fundamentally a communication disorder. People with autism all share a common set of traits, but the spectrum of traits combine form a constellation on a map, rather than a point on a linear scale. If you think of each aspect of human behaviour as a scale of 0-10, with 0 being 'this just isn't an issue' and 10 being 'it couldn't get any harder if you tried!'
People with autism all have difficulties in interpreting the words and actions of others, and communicating in a way that others understand, whether verbally or by actions. The degree to which they have difficulties varies according to their unique profile. For example, someone with ASD could be so aware of the small nuances of behaviour, tone, body language, and vocabulary that they notice even minute fluctuations and are 'over aware' of communication style, which can cause difficulties because they get very anxious and try to compensate for perceived hostility when there is none intended, such as when someone has a slight headache so is slightly short in tone. Conversely, someone with ASD could be pretty oblivious to changes in tone, etc., so they will seem insensitive and ignorant when someone is trying to communicate that they just want to be left alone. Someone with profound needs may not be able to interpret language (either verbal or body language) at all, and therefore nothing you say will prompt an appropriate reaction.
Similarly, in terms of outgoing communication, some may over think and be extremely verbose, trying to cover all bases and make sure no detail is left out, some may not realise that you don't know what they know, so will give scant information and you have to try to 'join the dots' yourself, and those with profound needs may not communicate in a way that can be understood at all.
In terms of behaviour, typical people learn by cause and effect. For example, a 9 month old suddenly starts pushing their cup off of their highchair. Mum hands it back, a game has started. An autistic person may not see the correlation between their behaviour and the consequence easily (DD1 will pace and harass me if we're going out, and when I finally snap that she needs to give me space she says 'why are you always angry with me?' because she can't understand that pacing backwards and forwards, reminding someone that we need to leave soon 100 times, etc., is irritating). A person with profound needs may not understand that pushing hurts, that throwing something can cause injury, or that some actions have permanent consequences.
Then, add in dysregulation. That's when the nervous system takes over in the 'fight or flight mode'. Adrenaline floods the body. The digestive system shuts down and all the blood flow diverts to the muscles, ready to fight or run. Often, people with autism have really inadequate interoception. That's the ability to know how you're feeling. Many emotions are felt as anger. There's a children's book that describes that anger as a red monster. When the red monster wakes up, the eyes get really tiny, so the person can't see. The ears get really tiny, so the person can't hear. The mouth gets really big so all they can do is scream/shout.
The combination of poor interoception and high adrenaline is pretty toxic, and it can lead to physical behaviour that is injurious to the person or those around them. But it isn't 'behaviour' in the true sense because often the person has completely lost control and they aren't choosing their behaviour, they are simply responding to impulse. Their inhibitions have gone.
It doesn't mean that it can't be changed. Adapting the environment to reduce overwhelm can help. Learning to recognise the signs of impending doom and getting in early with distraction and redirection. Giving clearer signals for transitions or changes of plan. Reducing demand. Reducing stimulus (quiet, dark, cool, etc). Sensory input (e.g. movement or stillness, soft fabrics or knobbly surfaces, something to chew, something to drink, etc).
Social stories can help people with some level of language understanding.
However, if you haven't managed to avert crisis, containment is the only sensible solution until the crisis has passed.
So basically, it's a combination of not understanding what is communicated, not understanding how to communicate, not understanding the consequences of actions, and not being regulated.
The difficulty is that some people with autism absolutely can understand the consequences in theory, but they can't usefully use it to shape their behaviour in the moment.