(I got my first dose of reality in Chiropractic School. It was obviously placebo, but placebo was just not talked about. Selling out is so common. I'm not talking the cute TV version on of selling out, but the real people who con the uninformed, and vulnerable out of their money daily. There's a lot of denial in so many professions.)
1. They've somehow participated in vote manipulation.
2. They were associated somehow with a sockpuppet account, which was itself banned.
It's probably better to stick to vouching for good comments that you think shouldn't be banned, and less on analyzing the reason for a ban (as you say, they can always just mail hn@ycombinator.com to find out what's going on).