Its a symptom that anxiety and fear causes, its also something that happens when someone gets "triggered". Not triggered as in angry triggered as in something sets them off to either stem, or tic, or self harm or whatever it just depends on the disorder. Adrenaline causes it to course through your body. Moving can help cause the shaking to decrease or stop it, but everyone who gets it has their own way to stop it if it happens to the point they have a way to end it.