I find it rather curious, and perhaps a bit puzzling, that in terms of petty insults used by internet trolls, "autism" has pretty much replaced the older insult "retard" (which in itself pretty much replaced the even older "nerd" and "geek"). And yes, "autism", not "autistic". (Although the latter is also of course used, it seems that at least 90% of the appearances of the insult use the former form, even when it makes no sense grammatically, or even if the sentence needs to be phrased awkwardly for the word to fit. Somehow "autism" seems to feel even more offensive and provocative than "autistic", for some reason.)
Or more specifically, the use of the word is not so much puzzling. Trolls will always use whichever terms are most offensive. It's the incredible widespread of the phenomenon that's a bit puzzling. It seems that no troll uses the older insults anymore, and almost every single one of them uses "autism" now.
Go to, for example, YouTube and search for almost any video that shows some kind of "nerdy" hobby (especially if it's not very mainstream), or any person behaving in a socially awkward (ie. typically "nerdy") way, and especially if the video has a significant amount of views and comments, I can 100% guarantee you that you will find the word "autism" in the comments. Moreover, you will probably not find too many of those "older" insults (such as "nerd", or even "retard".)
Perhaps the word "retard" has lost its "potency" as an insult due to overuse, and perhaps people feel that "autism" is a much more powerful and offensive word, a word that's much more provocative and controversial. (Not to talk about the much older insults "nerd" and "geek" which, by now, have lost their negative connotations pretty much completely, especially the former.)
I wonder how this new insult got so widespread. It has essentially reached a meme status of trolling.