Hey guys, just wanted to chime in, because the answer was never really stated.
Before I go into the answer, I'll just note I don't use, nor condone using, the insult nor anything used to label people in other situations as insults (gay, retarded, etc.)
So anyway, the insult of being called autistic, while it has devolved into meaning the same as "retarded", "idiot", "%%%", etc, didn't initially mean it. It was usually used for people who were shown going "too deep" into a topic. One of the more focused stereotypes of people with autism is their obsession with a specific topic, with the obsession going up to or beyond the levels of a person called a "geek." The second focus would be the tantrums and perceived overreactions to things going out of a person with autism's usual habits or expectations.
So, that person giving an in-depth complaint about a 5 minute fan-made cartoon would be insulted as autistic.
The person questioning the use of specific verbs in a television show would be labeled as autistic.
The person focused on stuffed toys from the 80's can be insulted.
Basically, if you were perceived as overreacting to a tiny detail that is usually negligible, or are focusing on a topic more than others, you can be insulted by trolls with it.
But as I said, the insult is now just thrown around by any person who thinks they are tough on the internet for any reason.