could be scared/cold/sick or any combination of those. Does it happen at any other time or just when being touched? How long have you had the bird and what is its history?
How is it shaking? Is it an up/down in/out type motion? That could be a regurgitation based behavior where the bird is trying to regurgitate or asking to be regurgitated into. Could be shivering if it's quicker and all over. Could be something else. Really depends on what it looks like.