Thanks for sharing.  Following the comments on youtube and the wikipedia page [1] I found this scientific analysis [2] about probable causes of the accident.  My layman summary: rising pressure of respired gas at high depth (about 26 atmospheres at the bottom) increases the respiratory resistance both in the diver (lungs, larynx) and the diving equipment.  There is a limit to exhaled volume flow-rate called &quot;effort independent flow&quot; where compression of air-ways due to exhalation keeps air-flow from increasing further.  This limit lies at lower flow-rates when breathing high-pressure gas.  Physical exertion causes increased CO2 production by the diver which needs to be matched by correspondingly increased air-flow for CO2 removal.  
Once point of effort-independence is reached, lots of respiratory effort is wasted, contributing to further CO2 production.  This causes a downward spiral of increased CO2 accumulation in the diver that could only be broken by switching to alternative breathing equipment that reduces respiratory effort (i.e. open-circuit gear).<p>Seems that close-circuit diving equipment is generally more risky to use [3] due to difficult to detect and non-correctible failure modes.<p>[1] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;David_Shaw_%28diver%29" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;David_Shaw_%28diver%29</a><p>[2] <a href="http:&#x2F;&#x2F;www.ingentaconnect.com&#x2F;content&#x2F;asma&#x2F;asem&#x2F;2007&#x2F;00000078&#x2F;00000002&#x2F;art00001" rel="nofollow">http:&#x2F;&#x2F;www.ingentaconnect.com&#x2F;content&#x2F;asma&#x2F;asem&#x2F;2007&#x2F;0000007...</a><p>[3] <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Rebreather_diving#Risk" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Rebreather_diving#Risk</a>