No one knows. Gbs occurs when the bodies immune system attacks the lining or myelin of the nerve cells in the peripheral nervous system. Although it can be associated with things like following surgery or after certain infections, for example it has been seen following campylobacter infection which causes diarrhea, the true cause is unknown.
Autoimmune. Guillain-barre is caused by inflammation of the peripheral nerves - the immune system attacks your own nerves. It starts in the arms or legs in the form of tingling or weakness, eventually affecting the respiration muscles if not halted (whole body paralysis can occur). It is often preceded by an infectious illness (respiratory infection or stomach flu). Molecules of the body attacked as foreign.