Maybe that's it. Maybe it was Ubanga/Holder who dispatched the "whistle-blower" to the Republicans in order to short-circuit such a scenario.
I think the intent was to have Petraeus testify by the administration thinking his affair was 'secret'. I do not see how the Republicans could have known if there was any there - there based upon a 'whistle-blower's contact. Hence Cantor went to the FBI director... Well next thing Petraeus resigns with the apparent expectation he would not have to testify.