Regardless if Wandy didn't know he was hitting him on the back of the head or not, or meant to hit him on the back of the head, it's still a serious problem. It's like if you break an ambiguous law, not knowingly doing so. That doesn't make you innocent just because you weren't aware you were doing it. It shouldn't take a person getting seriously hurt from getting hit on the back of the head for them to take this serious.
From a reffing standpoint, I can understand the ref wouldn't pull Wandy off of Cung because he was hitting him on the back of the head because Wandy was on the verge of ending the fight. Also, it's in the ref's hands to protect the fighter from taking unnecessary damage when he can't protect himself. Is it okay for the ref to, maybe prematurely, end a fight in a TKO because a guy is getting hit in the back of the head?