That in itself doesn't explain the averaging design choice, but it does explain why they decided on the particular fly-by-wire design they have, where in normal law the stick inputs don't have a 1-to-1 correspondance with control surface deflections (like you have on a small airplane for instance).
Hence I'd say this is much more a design issue than it is pilot error, as both senior pilots were advising the right course but the interface design allowed the inexperienced pilot to silently override control. Yes, this junior pilot probably had too little training and reacted terribly; but what's the point of having multiple pilots if they apparently don't add any additional safety because coordination amongst them has to rely on non-technical means instead of being facilitated by intuitive interface design? Where's the iPhone revolution in planes?