Most probably, the same explanation holds for lake Nyos but, owing to the much larger size of the latter lake and the topography of the surroundings (the perched at lake, c. 1,000 m , is drained by deep and long valleys, sheltering several villages), the death toll was much higher, including the last victim stricken some 27 km downstream from the lake.
The carbon dioxide which erupted from the lake water is undoubtedly of volcanic origin, continuously seeping through the sediments of the lake bottom, most probably as a cluster of warm springs of CO2-bearing water. Due to the high solubility of carbon dioxide in water, a lake can dissolve a volume of CO2 more than five times its water volume. The stability of such a "time bomb" stems from the fact that CO2-rich water (e.g., soda water) is denser than pure water, as long as gas bubbles do not nucleate. The horizontal layering of the water column is due to the differential diffusion of CO2 and heat but, contrary to salt (which stabilises the thermohaline stratification of the oceans), carbon dioxide has a solubility that is limited by temperature, making the stratification intrinsically unstable. Thus, there is even no need of an external trigger (landslide, earthquake or heavy rain) to upset the stratification of the lake.