Here's the a thread on why some don't eat garlic and onions. Some says it heats up the blood making it harder to meditate. Others say it is an aphrodisiac, or inflames the passions of lust and anger. Another says it's disrespectful to breathe garlic and onion breath over reading Buddhist scriptures. Lastly, someone said that onions and garlic are avoided because of a Buddhist myth that it originated from body parts. I remember when my mom became pure vegan, she avoided shallots and chives as well.
http://www.forumosa.com/taiwan/viewt...p?f=98&t=36814