Regardless of the cause this is a sad event for all who knew the diver.
About the head injury...if there was a head injury that was pre-recovery the diver likely got slammed by surf in the shallows up against the rocks. (I'm not at home on the island at this time so I don't know what the water conditions were, for sure.) This is common at this site, particularly, when divers are in the bubble cave or toilet bowl areas which are cavernous, overhead environments in the surf zone. People often underestimate the risks at this site as it is so commonly dived by folks of all experience levels. Conditions can change quickly from calm and clear to turbulent.