It's called Stimming, and sometimes it takes form in inappropriate ways, but it's their way of coping.
Find the thing that is triggering the Stimming and remove it from the situation.

With my autism, I can't sit still. I have to be moving my hands or rocking or twirling my ankles. This may be their way of fidgeting. It is perhaps a comfortable movement for them that they have accustomed to.
Maybe you could reposition them or give them some fidget toys or textured items or moving items.
Do they seem to walk and crawl well?
I used to love to lay down with my snuggle blankets and soft toys. Maybe just try distracting them with various things to see what works.

So far the kids kitchen play set is working for a good part of the day. She seems to be walking fine from what I can tell. I did notice that one of her shoes is getting worn on a certain spot but not on the other. She still humps, scribbles, tippy toe walk when she's excited and spins while flapping. At least it seems like the humping isn't as much.