You go to the local shop. You walk instead of running. Why?
Was it a concious decision, with explainable reasons? The benefits of running for most people are clear, so the only reason not to is because there is a reason not to. Perhaps what you are getting from the shop requires you have the energy to make use of it when you arrive back home. But it doesn't seem that people think like this at all. A child might always run to the shop, just by themselves and they run for the same reason most adults opt to walk: it's simply their nature and no one questions it. Questions about such things seem awkward and pointless, it's just what you do- you don't need to explain yourself. Unless you are Autistic.
It feels like most things I do are questioned and it used to be that I just didn't have answers because I didn't think about them. Being constantly questioned forces a person to think and look inside for answers. I sometimes wonder if myself and others I know are naturally high on self-reflection or if it is a common experience that makes it so. Asked "why do you pull faces at random?" I have to enquire about what faces I am pulling, unaware that I have done so. The first instinct: say something socially acceptable even if wrong. It takes a while to resist believing it myself, so I can then start questioning it. I don't get 'stiffness', I don't have a 'tic' and it isn't hay-fever. It is a stim, I do it because I am distracted, annoyed and expressing a reflex like any non-verbal body language, except mine is shaped by my individual experience of my sensory input.
When I am a twelve year old asked that for the first time by another boy, even if I knew it I could not explain it. There would be consequences which I will carry for years and they will have forgotten a week later. Like other alienated adolescents, I imagined the day would come when I grew into a supremely able adult who could smack every such person to a sorry weeping heap and face no consequences. A reflection on the wrongs received, how they were perceived and experienced. That might seem an overreaction but children are not inherently virtuous: knowing that questions were awkward for me just meant they would ask them incessantly for fun, to see the reaction.
Even when something can be explained, finding the right listener is a factor in being understood and that unfortunately makes it pointless trying to explain anything to some people. There's no common ground on which they can relate.
There was probably once a time where an Autistic would be just as likely to run someone through with a spear as answer a question about what they are doing. Not for gain, or for fun or defence- but because it works as an explanation. The alarm, the pain, the dread of what happens next, that sense of unfairness and that there was no way you could have known. Experience shared has a unique clarity to it.