Anxiety meltdowns, or autistic meltdowns as I know them, are when you are so overwhelmed by what's going on around you and your brain can't process anything that your behaviors change and you sort of shut down. Some of the behaviors I've exhibited include age regression (specifically, the flat-out refusal to leave my seat at a concert after it was completed), stimming (handflapping, leg bouncing, humming theme music), and complete shutdown (no response to anything outside of me). In my case, this happens so my brain can slow down and start taking in the information it needs to again. Hope this helps ❤️