1. The only tweet I've ever seen from you is a promotional one.... and I've seen it three or more times. I shouldn't be able to remember a promo tweet from you. The item, sure, the actual tweet, no. Break it up.
2. You're offensive - to me or others. Doesn't matter if it's indirectly. I don't care if it was a joke.
3. There's no "you" behind the promo. It's great that you want to help others promote themselves. It's wonderful that you have something to sell. But if you feel like a bot....
4. I'm doing my semi-regular following purge and I have no idea who you are or why you're here. If you haven't talked to me or I don't think you're relevant anymore, you're gone.
5. You immediately DM me after a follow with a link and promo for your site/work/album/book. Don't be a salesman without actually taking the time to find out if I'm in your market.
Bonus: The #1 reason you're never getting a follow-back in the first place.
You follow, unfollow, follow... enough that I recognize you from how often it's happened. I don't auto-follow. If you want me to follow you back, interact with me. Don't be a creep. This method leads me to assume you're doing this in an attempt to artificially boost your following.