>>5010SA tended to be quite cliquey back in the day and people were a little obsessed with being 'in' and that's something that carried over the modern social justice crowd. Even though I liked some of jokes from there at the time, I really disliked that aspect of it. I think there was too much emphasis put on who said something and reputation - it didn't help that everyone's join date and previous posts were on display so it was far too easy for someone to be dismissed without even reading their post. I think there were a number of SA users who felt this way, including moot (which, I think, at least in part is why 4chan is anonymous) and they 'moved' to 4chan and took aspects of the culture with them, most notably the dumb or ironic sense of humour and image editing like the stuff in this thread. A lot of butthurt weebs also came to 4chan because SA was shitting on them, and I think even to this day, /a/ is a little more reminiscent of the old SA than most of other boards. Of course, early 4chan had other influences, like the usenet crowd, which explains I think explains attitudes about trolling and things and users have found the site from loads of other places over the years like ebaums, newgrounds etc. so 4chan culture emerged from a really weird mix.
[spoiler:lit]sorry if this post is shit because I'm drunk as fuck right now so I've probably misremembered a bunch of crap and this is totally wrong[/spoiler:lit]