The conditions at Nyos and some of Africa's killer lakes are such that lake turnover rarely ever occurs, thus the build up of dissolved CO2 at the bottom of the lake over long periods of time.
Part of this is from the lakes location on the equator that leaves surface temps very stable year round, there are no cold fronts to cool the top layer of water to make it sink.
Another condition are the high walls characteristic of volcanic lakes that keep keep the wind off the surface of the lake and thus from turning the water. The last condition is the extreme depth of the lakes which allows for large amounts of dissolved CO2 to accumulate and remain "bottled" up under pressure.
Every once in a while a landslide into the lake or something else triggers a lake turnover and huge amounts of CO2 are brought to the surface. CO2 is denser than air so it travels along the lowest points through valleys away from lake.
http://www.geology.sdsu.edu/how_volcanoes_work/Nyos.html It had been known for years that the water in Lake Nyos was extremely enriched in dissolved CO2. The lake overlies a volcanic source, which appears to release CO2 and other gases. However, most of this gas does not escape into the atmosphere, but rather dissolves into the bottom waters of the lake. At a depth of over 200 meter, the sheer weight of the upper lake levels exerts considerable pressures on the bottom waters. This confining pressure allows CO2 to dissolve into the bottom waters without escaping to the surface, in much the same way that the cap on a carbonated beverage prevents CO2 from bubbling out of its container. At a depth of 200 meters, water can hold 15 times its own volume in CO2. It has been estimated that every liter of water in the lower part of the lake may have contained between 1 to 5 liters of CO2!
