It is a comic about how some vegans have an overwhelming desire to make it known they are vegan no matter what situation the are in. In the comic a person is doing something completely unrelated and a vegan invades their personal space to say they are vegan. And thus generalisations are made about vegans much to the annoyance of normal vegans.
Thanks. I understand it now.
But fans of any kind broadly do the same, don't they ? (Kinda kidding )