Why do some mobs suddenly grow courageous?

[unsunghero]

Certain mob types run when low health....until they don't. I understand a mob will fight to the death if it's rooted, or if another target is critical low health nearby

But sometimes I have seen a mob that would otherwise run, randomly decide to grow a pair and fight to the bitter end. Nothing around is critical low health, and the mob is not rooted

Why is this?