Always sad when a fellow diver dies while pursuing this sport. However, like mentioned there is nothing that stricter regulations could have done to prevent this situation.
I happened to stop by my local dive shop this evening and we were discussing the incident. I was told it was a hammerhead speargun. While I don't know the source of the information I trust it. Having dealt with a lot of in various states of maintenance There are a lot of probable causes. Certainly a a finger on a trigger is the most likely but a lot of these style guns have issues with the shaft not engaging properly. This is just speculation and we can beat our heads against the wall trying to figure it out and never get the answer.
Hank I know a couple friends that come up with the flopper hanging off their finger like that. For the life of me I can't figure out why. There is zero advantage and like mentioned incredibly dangerous.
Stay safe guys.