Vegan here. and mostly for the environmental repercussions of the meat industry and because meat is dirty as ****. Also dairy, I discovered after going vegan for a month, actually makes me feel like crap when I'm not addicted to it. It also increases the amount of lactic acid (the same stuff that working muscles create) in your body, causing you to feel muscle fatigue more quickly and makes you produce more mucus which decreases breathing capacity. Summary: DAIRY IS BAD FOR BIKING! MEAT IS DIRTY! BOTH ARE BAD FOR THE ENVIRONMENT!
As far as bikers being Veg, I think as mentioned before it has to do with us being more socially conscious in general.
p.s.- Queerpunk, can I come kick it with you on the east coast some time? Gender****, anarchist, bikepunx for life!!