A pouch of magma exists below the lake. This magma converts the water into carbonic acid by depositing certain amounts of carbon dioxide into the water.
The 1986 disaster was a result of a limnic eruption (commonly referred to as lake overturn). This is an extremely rare type of natural disaster. Small amounts of carbon dioxide gather together to form a large pocket until it suddenly erupts, releasing this CO2 into the surrounding areas. These eruptions have been known to cause small tsunamis as the carbon dioxide that rises displaces the water. It is believed, by many scientists around the world, that landslides, volcanic activity or any kind of explosions can trigger a limnic eruption.
To prevent another disaster, in 2001 a tube was installed to draw water from the bottom of the lake to the top. This method allows the carbon dioxide to escape in small, safe amounts. In 2011, another two tubes were added to assist in the degassing of the lake.
Further reading and references: