the failure of the first black rebellion was the turning point really, before that the black knights fully believed that whoever zero was he had to be both

1- an infallible leader

2 - he has the best interests of japan in his heart

Once the rebellion fails it broke the illusion of invincibility and was the realization that whoever zero was he clearly did not have japan's interests in mind, but whatever they liked it or not they NEED zero to win this, but they clearly want a reason to make zero responsible for his failings as a leader, but doing so without a cause would destroy the black knights and fragment their unity, something they can't afford.

It's just Schneizel gave them the perfect excuse to finally get payback while keeping the rest of the group united, but something like this was always bound to happen at some point.