It's a property of the solubility of oxygen in water. While solids dissolve better in hot water, the opposite is true for gases. The hotter the water, the less dissolved gas it can hold. Fish need dissolved oxygen to "breathe" so when the water gets warm, there is not enough oxygen in the water to keep them alive.
If you want to know *why* oxygen dissolves poorly in warm water, here is an explanation from Lynn Bry at Harvard Medical School in Boston, MA: "Keep in mind that a molecule in the gaseous state has far more energy than a molecule in the solid state or in the liquid state. To a gas, cold water is peaceful and calm. With less activity, it's easier for a 'high energy' gas molecule to slip among the water molecules and stay in solution. However as the water is heated, the added bouncing around of the water molecules makes it difficult for a gas molecule to remain in solution. Since it already has a lot of energy, a hit from a passing molecule of warm water would knock it out of solution into the surrounding air."