>>31192786The TL;DR is that Ash-Greninja is the result of Ash's particular Greninja being a special snowflake and having secret, untapped powers. It's only because Ash is the kinda guy he is that he was able to unlock the power Greninja had inside, bringing out the Ash-Greninja transformation.
There's a bit of lore, as well, that hints that this form of Greninja has appeared in the past, and that it's purpose is to protect and save those around it (as
>>31192776 was talking about, Ash-Greninja can sense negative energy caused by Team Flare's schemes, and this was foreshadowed in the Ninja Village where the form is hinted to exist).
