They found some papers where children with autism or ADHD often were found walking a way that causes an “anterior pelvic tilt”, which makes your butt stick out more. I myself personally happen to have such a tilt while being autistic and yeah, my butt sticks out more. 
Fun fact: in Dutch ASD is “autismespectrumstoornis” aka “ASS”
This is exactly how this dogshit spreads. Same with the hatred laced in Tylenol jokes. I don’t care what undignified ways people internalize their ableism. Get this shit out of my face