So what if we are going the wrong way about Zygarde form change. What if its not ability triggered since the ability didn't pop out and its in fact triggered by moves sort of like Meloetta's relic song but instead of changing form after using core enforcer.
That would explain the sudden change from 10% to 100%. Not sure how the 50% form will work though.
But if this theory is correct that would be cool because you can use the only 10% form if you wanted.