Indeed, it dropped off the front page several times: http://hnrankings.info/4217596/ (the rank gets updated every 5 minutes, but at 11:55 and 12:20 there is no data point in the graph.)

Compare it with a more typical curve: http://hnrankings.info/4216152/

Article might just be very controversial, with lots of up and downvotes.

Not sure. Maybe it triggered some sort of filter because it's about pornography. Does anyone know if there's anyone you can contact about this? I've tried emailing the ycombinator address, but I'm not super hopeful for a reply.