Agreed, I bet there was a recent underground Earthquake that release a massive bubble of trapped CO2 into the lake, which resulted in the fish kill or a massive unseen algae bloom.
A pocket of magma lies beneath the lake and leaks carbon dioxide (CO2) into the water, changing it into carbonic acid. Nyos is one of only three known exploding lakes to be saturated with carbon dioxide in this way, the others being Lake Monoun, 100 km (62 mi) away SSE, and Lake Kivu in Rwanda. On August 21, 1986, possibly triggered by a landslide, Lake Nyos suddenly emitted a large cloud of CO2, which suffocated 1,700 people and 3,500 livestock in nearby towns and villiges.[2]
Source: Wikipedia