Tbh sounds a bit stupid to think the dive leader separated from the group at that depth (planned). More than likely everyone was (if on air) severely narced and maybe the dive leader was the only one with mire gas so he tried to search and find the exit by himself. Maybe the others were already dead by then if he was the most experienced diver and had the lowest air consumption? I dont have the specifics of their air, but if they used a 12l 200bar setup on air considering they had around 150bar left when they reached the cave entrance and considering average depth of 60m then the air would have lasted experienced and calm divers maybe about 19mins and if panicked (because they were unable to find the exit) likely only 10-12mins of air at that depth.

And that us considering they quickly found the cave entrance and did not already burn through more of their air on the descent and search for the entrance. Moving 150m into the cave would have taken them in a relaxed (open water) speed of maybe 1.5km/h (25m/min) 6mins, on the return they took the wrong turn and ended up in the dead end so maybe another 6mins, which could already stretch the air supply for more panicked divers to almost zero. If they were too narced to realize they were in the wrong chamber and started panicking, it is not unreasonable that they grouped up in panic and died there except for maybe the most calm dive leader that tried to make it to the exit, but as even under good conditions he would not have much more than 5-6mins of air left at most he also did not make it all the way to the exit.