Part of the meaning is derived from the idea that these are not just nuts, they are nuts who exist in extreme political wings. wing + nut, in the sense that wing is the modifier of nut. I don't doubt that it was used in People's Park, but I'm sure it was used well before then.

I'm not sure I could narrow down what really counts as wingnut behavior, but gordon's definition is accurate enough. In my experience, it's not just an anarchist term, nor are wingnuts necessarily anarchists or not. There are wingnuts representing just about every political idea you've never heard of, and some that you have.