Both bad encounters I've had, the shark was after the fish. At least this is what I believe. I've had a very large Female HH swim around gracefully with her pups without as much as a glance at us. All I can say is that some people get use to diving in these situations and find it normal. I feel that once this happens, you're letting down your guard. Why don't we see hunters in Africa going through the jungle fighting with lions for their prey? Shooting antelope off of a Lions back and then rushing it and fighting it for dominance? Crazy huh? What's the difference? At least on dry land we have a breath of a chance....
Difference is that when you shoot a fish off of a sharks back it normally spooks the crap out of them and the only thing you have to deal with afterwards is the fish. I imagine if you shot a animal off of a lion he would probably spook too. If he rushes you you are in big trouble....if you shoot and would the lion and he is still charging you are in a world of shit because unless you brain or spine him he is going to make minced meat out of you.
There are some people who hunt lion and believe the only sporting way to take one is within charging distance and only with the use of a large bore open sight double gun. You stalk close...stand up and elicit a charge and then bring it down with one of your two shots....it is standard practice to hold another two rounds in between the fingers of the hand that holds the forearm of the gun in case you need or have time to reload and give it two more. I read a story by a PH in Africa (Ruark) that had a lady do it this way and the lion expired literally at her feet. He had another story of a guy that did it and he missed all four shots. The PH (professional hunter) managed to get one in it in the lungs and the lion broke off the charge and died in the bush....dude pissed his pants.
But I think the real reason people charge aggressively at sharks that are too interested in their speared fish is because it works and it keeps the shark from not only eating your fish but from becoming even more aggressive once it eats your fish and then starts to look at you for something else to chew on.