Dead fish don't emit sulphur smell. They emit dead fish smell. Volcanoes emit sulphur which when emitted in water can kill fish. Perhaps there was a volcanic release of sulphur that killed all these fish.

The Salton Sea is a man made body of water. A dike broke about a hundred years ago and it flooded this area with water. Maybe the water has now percolated far enough down in the ground to reach the magma that's a mile or so below the surface, resulting in boiling mud pots and sulphur releases.