This is pure speculation, but I believe we can blame J. Edgar Hoover for Mr. McCabe's firing.
Back in the day, that's the 1920's, 1930's, 1940's, 1950's, 1960's, and only ending in 1972, Hoover ruled the roost at the (Federal) Bureau of Investigation. He was a very political leader, using information to protect his position and advance his issues. He had strict rules for his agents, because he was the one who could bend the rules.
My speculation is that the FBI culture retains that dichotomy: rules on the one hand, leaks to advance the agency or leader on the other. And that seems to be what happened with McCabe. He authorized a discussion on background to, he says, correct erroneous information reaching the public. He claims it was something often done, but it seems to have also been against the rules. So when OIG people interviewed him, he was caught in the middle, not admitting to something which was okay by FBI norms, but not the rules.
Again, speculation, but to me the culture of an agency lasts, and lasts.