In my opinion, I think the geo-myth; "The Lake and the Valley" describes a dormant volcano eruption. The volcano must have seemed to erupt many years before the people settled into villages, and used the land. The lake in the middle of the valley was probably covering over the latent magma, and molten lava. The grains that were thrown in the lake was probably growing into a sort of water plant, growing over the magma, preventing the lava, etc.
from erupting. Since the leaders of the villages wanted to sell the surplus grains instead of sacrificing it to the "fiery dragon" at the bottom of the lake, or better known as magma, the plants couldn't cover, and protect the lake from the coming magma, and lava. Since the volcano probably erupted many years before, countless times the area, animals, plants, and other creatures were most likely used to the heat, and temperature the lava, and magma gave off. The villagers weren't, causing them to die of heat, and temperatures that their bodies couldn't handle. Ca yvon wasn't affected because he was on the edge of the valley while he heard the loud noise, and his wife was not affected because maybe she was working in a room, underground her house, like a basement. There could be many, various explanations that might clarify this geo-myth, I might be right, and I may be wrong, but I think this is a very plausible explanation for this geo-myth.
There are many other geo-myths that explain this same type of situation.