How does Confusion in the Ranks trigger itself upon entering the battlefield?
Triggers look at the board state immediately after the spell resolves and moves from stack to battlefield.  The "event" is object of given type moved from stack (or other zone) to battlefield.  The state of the board is checked for triggers that see that event immediately after the event, and so there it is.  I guess there isn't really a better answer than, the comp rules say so, so here it is!


  603.6a Enters-the-battlefield abilities trigger when a permanent enters the battlefield. These are written, “When [this object] enters
  the battlefield, . . . ” or “Whenever a [type] enters the battlefield,
  . . .” Each time an event puts one or more permanents onto the
  battlefield, all permanents on the battlefield (including the
  newcomers) are checked for any enters-the-battlefield triggers that
  match the event.
  
  603.6d Normally, objects that exist immediately after an event are checked to see if the event matched any trigger conditions.