‘“Reeeeeeeeeee” is the sound that a social justice warrior (sjw) makes when they see or hear something they don’t like, such as the truth.’
It derived originally from the sound an African Screaming Frog makes when it’s startled, threatened or frightened. I.e.: it ties into the Pepe theme.
https://m.youtube.com/watch?v=7Jm_eUQSYAk