I just happened to start my own gag @reply bot last night, https://twitter.com/AssHobby, and quickly hit this same problem. I tried to get clarification on what I was doing that was different than a bot like StealthMountain but of course just got a canned response that answered nothing. It really can't be anything other than followers count or special exemptions. Maybe drastically reducing the frequency such as sublimino is doing would help keep it off their radar. But a definitive answer would be nice.