What causes companions to sometimes die while others they are just incapacitated and revive when you've killed the last NPC or mob?

In my experience, the only time a companion of mine has permanently died instead of just being incapacitated (i.e. going down on their hands and knees to catch their breath) was when I accidentally hit them with friendly fire when they were down. In every other case, the enemies will not target them for an attack to kill them.

That is how it is supposed to work. Companions should yield when low health but if you hit them they die. Big problem if you use spells since they almost all have an AE componenent.

When a NPC goes to his/her knees, all enemies will stop attacking until they are back on their feet. The only way the companion then can die is if you hit them OR an enemy hits them when trying to hit you. Example: The companion is on his knees between you and the enemy and the enemy hits the companion when aiming for you..