As String has already said it is not good to speculate. Please keep the guesses to yourselves until any facts emerge.
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String:No cause known, all above is just supposition, the only news reports are saying 2 on the surface in distress and sank, no sign of rapid ascent or anything proven so best not to speculate.
pufferfish:Looks like there is something peculiar to Scapa Flow
From DAN Diving Fatalities 2004
Group................DCI Incidence per 10,000 dives
Liveaboard............................0.0
Shore/Day Boat......................3.0
Scapa Flow............................10.7
I have heard cold water, deep, and current as reasons
What about other factors? Fitness, experience of divers , multiple fill stations or only one available in the area
String:Yes scapa is cold (northern waters), dark, and deep- some wrecks are very deep in there.
Add that to the fact Scapa is generally visited as part of a multi day diving trip or liveaboard due to its remote location and you can find yourself doing multiple 30m+ dives in a day. In short, its gruelling.
(note my comment is not meant to judge what happened in this case, just stating something abour scapa!)