My thoughts on the correlation:

1. Veganism and fixers are both popular in urban centers, among young, liberal people, etc. They share meme turf, so are more likely to intersect in a single mind than, say, curling and playstation.

2. Veganism is somewhat of an extreme in that it necessarily becomes a rather significant part of a person's life. Riding a certain type of bike isn't necessarily as intrusive on other aspects of one's lifestyle as veganism is, but as a meme it does seem to embed itself pretty deep.

3. Most vegans I know went from a normal diet to vegetarian to vegan, just as most fixers had the freedom to coast when dad's hand let go of the seat. In other words, for the most part both are individually evolved lifestyles.

4. Vegans are notoriously holier-than-thou. There is certainly a significant portion of fixers who are, too.

Basically, vegans and fixers strike me as the kind of people who go head-first into particular things, and who tie particular ideas so strongly to their personalities that they become hard to separate. As a vegan who doesn't coast, and as a skeptic, I do worry sometimes that my mind is wired like a religious nut's. Fundamentalism sucks any way you slice it. Sadly, I think it helps explain the correlation of vegans and fixers.

I've rambled, but I've been thinking about this lately and thought I'd share. YMMV.