"REE" has had a long history.

Its origins are from a video by myg0t 2, popularized by Newgrounds in 2004. Back then, people wrote "EEEEEE" or "EEEEEEK" to represent the scream. In 2006, there was the birth of "REE". Most people thereafter, referred to the scream as "REE".

Videos of frogs have been uploaded screaming since around 2009, but their sounds were not characterized by "REE" until later on. People used "WEE" or "WRYY", which is itself another meme. You will not find any comment using "REE" on those videos before 2015.

"REE" made a huge resurgence in 2015. The earliest use of it on the /r9k/ board coming at the very end of 2014, with a Pepe meme, a clear reference to the frog screams. It is unclear if there is relation to myg0t 2, however considering how influential myg0t 2 was way back when, the links to social awkwardness, and that "REE" was pretty much unheard of before myg0t 2, it seems plausible that there was an influence. The scream was used as almost a unifying slogan of /r9k/. It was also representative of a social awkwardness counter-culture (anti-normie), but it was not an insult of socially awkward people. Autism was not an obvious link at the time but I think it very quickly developed as other boards started using the Pepe meme, taking "REE" along with it (such as /pol/). "REE" then became an insult against autists and socially awkward people. There was a lot of resistance on /r9k/ at the time since some of the meaning of their mascot and meme, was taken and warped by the rest of the internet.

Nevertheless, here we are now. The "anti-normie" idea behind it comes from its original use on /r9k/ where, as I mentioned, it represented the entire counter-culture that /r9k/ embodied. Not being "normies" (socially adjusted people), is something that unifies their community and something they're almost proud of.