The cause of gas bubble disease is typically nitrogen super-saturation of the water. Super saturation is caused when water and air are put together under high pressure, like in a tunnel, pipe, or aquifer. If the inlet of the tunnel is far enough under water that no water is entrained with the water going down, then there will not be any super saturation. If water sucks in air on the way down, like when the inlet is just barely underwater but a whirlpool sucks in air, then the water can super saturate.
Also, if the receiving water is very deep, super saturation can have little or even absolutely zero effect (deep water holds more nitrogen, so it takes MUCH more nitrogen to be super saturated).
The water level at the intake and/or the outflow could be different (I have no local knowledge) after they breached the dams. Even something like a slower flow rate through the tunnel could be the difference between their eyes popping out, or not.
