why were the nuts nervous outside

While they desperately, the patients are used to the restrictions and the rules that are inside the institution; once they are outside, they are on their own with only McMurphy to guide them and that is a very, very scary thing for them. However, Murphy shows them how they can be "crazy" and still have fun in the outside world. They do not quite know how to deal with the fishing trip, but McMurphy shows them how to fish and drink among other things so that their identity has developed in a somewhat positive way before they return to the institution.