What was your MP at the time? What Intervention Level?
When you were changing equipment, did your HP drop below zero? You could have experienced the known issue with negative hit points?
When you equip items that increase your hit points (either through direct HP gains, or via a trait), those points are added instantly to your life total. When you take that equipment off, the extra hit points are removed instantly. If your hit points are less than what is removed, you can get a negative value, displayed as a having zero life. You can walk around, talk to people and do pretty much anything with zero life. If you get any type of damage while having the negative value of hit points, you instantly die and intervention DOES NOT trigger (even if you have a full MP bar).
Sometimes, if you swap out equipment while taking damage, the game treats the damage as hitting you without armor and at the lower HP threshold, which if is below zero, see above. This hiccup is not easily reproduced and is likely due to a timing thing. The game has to be paused an the exactly right nano-second for the game to start processing the damage, but not updating the graphics???