You could say Baine may have started it when he defied and released Jaina’s brothers however that is not to say the rest had rebelled at that time. They may had misgivings about sylvanas but never openly challenged her or disobeyed her and actively support her with troops. They only openly rebel do that after Saurfang and Thrall save Baine(with alliance help)
Thinking your warchief is a bad guy and actively doing something about it are two very different things.
Vol’jins rebellion didn’t start until he recovered from attempted assassination by garrosh. Even then he spent time playing around with monks fighting the Zandalar before returning to the Horde.
Before the Assassination he was following Garrosh’s orders just like the rest.
Did they openly speak up against her or disobey her though? No it was all hushed conversation. Even Lothermar feared that his people would support Sylvanas if he stood against her.
On the flip side take the Alliance where not only does Tyrande openly challenge Anduins decision she disobeys him and takes her forces out of the Alliances hands, Genn also does this. They couldn’t do this if Anduin was in absolute control of the Alliance(though he acts like it sometimes).
Even after the war has ended Tyrande still refuses to agree with Anduins peace treatybut is still a member of the Alliance.