Sensory processing issues.
I have a theory. Hear me out.
My theory is that all autism stems from an abnormality in the way the brain processes sensory input. Aspies develop coping mechanisms to varying degress, ranging from some "overcoming" the abnormality so well that they do not report any problems at all to others having obvious symptoms remaining. They really haven't overcome the problem, though, their brains are just good at working around them.
This coping process, which starts at birth, in turn causes their brains to develop certain areas more than normal and others less than normal. Their brains soon become wired in an atypical way. Probably by the time they are a couple of years old, these become permanent, ingrained habits. As they grow up and try to figure out the world around them, Aspies begin to exhibit certain traits as a result of this strange wiring.
All other traits are a result of this. For example,
resistance to change: you have this set way of working out what is going on around you (remember, you cannot process your sensory input normally, so you are using a workaround). If your environment changes drastically, then your old workaround may no longer work. Confusion results. You prefer to stick to a routine and what is already familiar.
thinking too much: your brain has to do some extra processing for so many everyday activities, so you get used to thinking all the time. It seems like the right way to approach everything.
sleep problems: with all this thinking, it is hard to settle down to sleep.
social interaction: this one is complicated. A lot of it seems to come from the fact that you are expected to act illogically. To talk for the sake of talking, rather than having something to say. This goes against the Aspie thought patterns. Also, there may be visual cues we should be picking up, but if we do not know where to look for them, we will miss them. Not knowing where to look could be a result of coping with vision abnormalities.
This is just a short list off the top of my head. I haven't really finished this theory. Just remember, each Aspie is different, because the exact level and style of coping is different for each person, but certain patterns emerge, so we see many traits in common.