In episode 36 of the Pokemon XYZ series, it shows Professor Sycamore/Platane approaches Ash and Co. and tells him about "Bond Phenomenon" occurred with Ash and his Greninja (which they explain in the episode) The overall "form change" of Greninja is probably an artistic liberty of the animators to show the "bond" between Ash and Greninja, whereas "Bond Phenomenon" is referencing Pokemon Amie.