They're so in sync.

I cant wait for Ash to abandon him at Oaks.

Greninja here is going into a form called Ash-Greninja, a totally-not-mega evolution mega evolution which comes from magic of friendship instead of a mega stone (supposedly the form gives off mega evolution energy according to one episode). There were some foreshadowing stuff for that but no one really cares. The water shuriken henshin Greninja doing is a signal that the totally-not-mega evolution has been perfected, something which Ash took a little arc to do since his Greninja is too good for him. It contributes to the quality of the show because goddamn if that animation isn't cool as shit (also it gives Greninja a mega boost in stats and it becomes superfrog). Lots of fans have come to the conclusion Ash has Ash-Greninja instead of a traditional mega because then he'd have a mega ring and key stone and that would be baggage to address when he eventually ditches everyone for Alola.