The second movement began in August, the surge, I don't know why-any thoughts?

There is no active campaigns usually get ad about the visitors to 1 or 2 day (in the past 18 months).  My site is more of a scientific experiment, but it's nothing unusual.  Suddenly, one day, 50-75 traffic jumped to visitors a day.  Poured through Google analytics, Facebook insights, but can't figure out where.  All I know is coming to Facebook, not stand alone.  Application: chessroom

Any thoughts?