I think writing too many articles of certain types that violate ToS... but not necessarily vulgar, etc.
I received three warnings. All of them ended with something like, "If you continually do this, you will be banned from HP".
I asked a lot of questions about how to correct the first offending hub. The second one, I just deleted. I also asked how serious the banning threat is. No answer on that question.
But, it deflated me for a while. I got out of Amway when I learned that you never really "own your own business" because the Amway people can kick out anyone at anytime. It seems like a big risk to invest so many hours into writing hubs only to get banned.
The first hub had too many quotes. I replaced those with links. I spent days on that hub, only to rewrite it to a less valuable version.
The second one was a hub about how to date Ukrainian women using dating sites. I write on this topic (I have a blog) for one reason: to add some decency to the field. I believe a radical who started crying over my anti-Christian rock music hub flagged it for being a hub about porn. He commented that the blog had nude photos. It doesn't.
And, the hub had too many links, and was labeled "overly promotional". That is probably true. I put a LOT of links to individual blog posts.
The third came in the wave of accidentally flagged hubs. It was quickly corrected by the HP staff.