There are many reasons that an autistic child might run, self-harm, lash out or melt down. A child might be overwhelmed by their sensory issues, be terrified of being harmed (even by something that seems harmless to someone else), be responding to previous trauma, or simply not understand what is being asked or demanded of them. Or there could be another potential source of distress that is difficult for them to communicate, particularly if a child struggles to speak. My own autistic child speaks easily when he is calm but becomes much less verbal when he is scared.