I have a bizarre theory. If there is microbial life in that lake, it energy source would have to be heat from the earth. The thermophiles in our oceanic organisms that exist around these heat sources use sulphur in their metabolic processes instead of oxygen. The non thermophiles eat the thermophiles but use oxygen. The Thermofiles have Hydrogen Sulfide as a waste product. Hydrogen Sulfide is almost as toxic as Hydrogen Cyanide.
This hydrogen sulfide would accumulate at the top of the lake and below the miles of ice. The drilling fluid they are using is basically kerosene. Kerosene is lighter than water and thus water from the lake would rush into the drilling string and once this happens both pressure inside the drill string and the lake would be equal and in essence you have a dead well i.e. no longer flowing.
However if they drilled into high pressure gas the surface pressures would be about 4000 thousand psi. Unless they have a Blow Out Preventer on top of the well head they could be quickly overwhelmed by this toxic gas. In low concentration it has the smell of rotten eggs. In high concentration it immediately effects our sense of smell adversely. It can not be smelled in high concentrations and we do not realize we are about to die of asphyxia due to the inability of the red blood cells to carry oxygen due to the Hydrogen sulfide attaching itself the the hemoglobin in the cells. You go to sleep and die very quickly. It takes just 3 or 4 minutes to render you unconscious and death will follow very soon.
It is quite possible they were all overwhelmed by HydrogenSulfide Gas.
Wouldn’t they have been prepared for such an outcome?