Ask HN: What causes this phenomenon on HN stories?

This post titled "What's next Google? Dropping SMTP support?" suddenly dipped from #1 to more than #10 within minutes. Some say it's flagging abuse by HN posters with good karma, while others say it's not.

Someone linked the HN Rankings graph which seems to show rank vs. time. You can clearly see the huge dip here.

http://hnrankings.info/5717995/

The same happened earlier today and on a couple of stories yesterday.

http://hnrankings.info/5717739/

http://hnrankings.info/5715168/

http://hnrankings.info/5715889/

I am just curious what could cause such drastic drops? Other stories appear to have smooth and gradual graph lines.

There was a heated discussion about this yesterday.

https://news.ycombinator.com/item?id=5716010

Is it something secret? Why does it seem to happen to Google related stories mostly? If the mods don't want us discussion those stories why don't they just tell us?