I fully agree stuttering has little to no correlation to social anxiety, or even generalized anxiety. You could be extremely relaxed walking into a social situation, and suddenly a word of phrase deep in your subconscious triggers a block. My theory, shared my many who practice Neuro Linguistics Programming, is that stuttering is nothing more than a collection of triggers we accumulated to get a word out in times of distress. Over time, these triggers and responses become more and more embedded in our subconscious and become regular methods and patterns of speaking.