I think it happens due to:
1. A lack of situational awareness. This is probably the most common cause (i.e. not checking the SPG, not knowing how much air you have left at all times).
2. Buddies not working well as a team / buddies with poor situational awareness. If an LP hose happens to blow underwater, the gas in the tank could drain very quickly. If a buddy is not within sight and paying attention, it could end badly.
2. Inadequate dive protocols/training.
- The SPG is inaccurate at extreme ranges, especially below 40bar. It would be better to call the dive at 40bar regardless of depth.
- The concept of minimum gas is not widely understood. This is the amount of gas that will get you and your buddy to safety doing all necessary stops along the way, should one diver have an emergency situation. "Surfacing at 50bar" is inadequate if you are diving to 30m.
- Doing bubble checks before descending is not commonly practiced among divers here. Thus if you do have equipment that is in early stages of malfunction, it goes undetected until things get more serious. This is an important drill as I have personally detected bubbling from the SPG, first stage body, hose connections, tank valves and rear dump valves that were quickly fixed because they were detected early. At minimum, you are aware you have a minor problem and you have the option to continue but be watchful if the leak is minor, or call the dive if it is more serious.
- Doing a flow check and being able to reach your valves should be mandatory whether diving in single or double tanks. In this way, if you are in doubt, you can always reach back and check your valves.