The wound — usually invisible to you, often invisible to them — that's triggered by a perceived slight, failure, or threat to their self-image. Disproportionate hurt that quickly converts into rage or withdrawal.
You tactfully mentioned a small mistake in their slideshow. Forty minutes later they're cold, sarcastic, and bringing up something you did wrong three years ago. The mistake landed as an attack on their worth. You didn't see the wound — but you're going to see the response.