I've yet to shoot a fish in the known presence of a bull shark or any reef shark. But they have a way of coming in out of nowhere once a fish is hit. It's amazing how fast they are.
Exactly, let's all end the speculation, chances are very good he had no idea that shark was in the area. When you spearfish sharks are always a possibility, attacks are rare but crap happens and any spearfisherman worth his salt goes in knowing he is taking a risk, albeit a statistically small risk. The guy was doing nothing illegal and nothing many of us haven't done hundreds of thousands of times without incident. This wasn't the sharks fault, and probably wasn't the divers fault either.