I'm not sure of the origins, but being 'in bits' or 'in pieces' implies (to me) the end result of 'going to pieces'. It's a more prolonged affair. You 'go to pieces' and end up 'in pieces'. I think it has developed to give extra emphasis or weight to the condition, so it's more emphatic.
This post was last modified: 29-09-2012 05:27 PM by rshdunlop.