It is called a "Social Ban". It is used to automatically limit bots and gold sellers from taking part in the game economy. It sometimes has false positives and catches innocent players. Create a support ticket, tell them you have received an incorrect Social Ban. You will likely get an automated response or something unhelpful at first. Follow up on that same ticket and say, "This is not helpful. I need a Social Ban removed." If you enter multiple tickets, they will all sit at back of queue. You want one ticket, and follow up on it so it gets bumped in priority and reaches support staff with power to remove a ban.

And, no, they will not tell you what you did that triggered ban. They are coy about this because if they tell you what not to do, it gives bots and gold sellers a blueprint for how to avoid bans.

@Dagoth_Rac
If this is true it is really weird. I'm not a harvestor not a flipper, nor do I have unplayed mule toons. I only sell between 250-500k a month in my guild stores and only upload stuff to sell every 2-4 weeks on average. I also am have never sold gold, done carries, and rarely even buy things from traders....I've been in game over 4 years, and log in almost daily to at least do my writs. I haven't a clue as to why I would be flagged as this.