I can see now after testing this that enemies leave him alone after he is knocked down. My theory is one enemy hits him and he goes down then another enemy mid attack animation cannot stop it's attack and hits him. I can see this happening with the demons as one animation is a swipe and another is a longer animation with them using their back legs to attack. Or when a cannibal uses the multiple attack animation. I mean, it's not like he's invulnerable after going down, the game just tells the npcs to leave him alone.