Sometimes: 1. It's their ego and identity wrapped up and it's a threat there. 2. They've been gaslighted so much that the disagreement is poses as a derailmen

Sometimes: 1. It's their ego and identity wrapped up and it's a threat there. 2. They've been gaslighted so much that the disagreement is poses as a derailment (This is when the disagreement is disingenuous) I'm sure there are other reasons as well.