Can't say I've seen it before and I just read a Wikipedia article linked from HN last night[1]. I'm guessing there's a threshold of visits before their software flips that switch.

My interpretation is that it's a nice way of staying "Don't argue about the content of this page here, do it on your own message board".

[1]https://news.ycombinator.com/item?id=5722458

Edit: just saw johnduhart's reply.