Architecturally, we are emotional creatures first and only occasionally have fits of reason. All human perception gets processed first in the limbic system. The emotional center of our brain colors everything we perceive. blog 
Photo of three 1980s vintage Lego spacefaring mini-figures from left to right: yellow suit holding a gray and red raygun, red suit holding a hand-held radio, and white suit holding a wrench overshadowed by a Lego Tyrannosaurus Rex looming out of small plain box.
There is fear during the incident—some imminent threat. Our brains behave differently under threat. We schedule the retro separately so we can reflect in a different state of mind. However, there is a different quality of fear in the retrospective meeting. We must now face consequences of the sacrifice decisions made during the incident.
As facilitators we are responsible for the psychological safety of everyone in the room. We aim to create an environment where participants feel safe to disclose uncomfortable details. When blame surfaces in the retro it takes everyone into a new incident—we're back in active fear. The facilitator's own fear will surface too. The open conflict has happened on our watch.