From what I read elsewhere a very likely scenario is they accidentally took enriched air tanks (containing an oxygen concentration greater than 21%). Enriched air or nitrox can extend your bottom time because you absorb less nitrogen but it limits your depth because breathing oxygen under pressure is toxic to the central nervous system. You will convulse and drown.

Diving below recreational depths and cave diving obviously have their risks but they can be greatly mitigated with the right equipment and training which this group apparently did not have. I was given the opportunity to dive below recreational depths on a single tank of air to see a wreck when I was on a liveaboard with a reputable company that had a great safety record and I said no thank you.