You'll most likely find it was either a Sch or Smn Pet, for some reason if your Sch or Smn place their pet within a certain range (Possibly in or on the line) of the 'hit box' then the boss will adjust, moving so the pet is no longer in/on the box. This has happened to me in both Titan where the boss even moved to the left of me at one stage and also the final boss of AK where they tend to attempt to stuff their pet between the boss and the statue so the boss will adjust as well.