I’m learning that people listen to the sound of your voice, not what you are saying.
Don’t play the victim. At all times, even under intense questioning, seem confident that it’s not your problem. It would be worse if they were muttering without making you aware, as you have no opportunity to put your case – you get a black mark and don’t even know it. That happens all the time here.
Play the hero. Perhaps suggest to management that as an expert in this area, you cast an experts eye over the problem. Expose why it always worked when you ran it, and what he has done wrong. But give him a way out? You could say it is an understandable error because he wasn’t as experienced. Offer to design new processes to avoid these problems.