Any time you want to question something a person did or didn’t, phrase your question in terms of “how” or “what.”
Don’t ask “why,” especially when working with individuals who tend to be a bit defensive.
I’ve noticed that a “why” often pushes them over the edge—they feel threatened, or they feel their abilities are being brought into question. Defensiveness is usually a means of avoiding accountability and getting the other person to back off.
The “tell me more” invites them to engage in a conversation.