The bear fell because it slides to the surface of ice due to lack of friction.
One of these theories is that friction causes the liquid layer of water to form on ice. Friction is the force that generates heat whenever two objects slide against each other. If you rub your hands together, you can feel them heat up. That's friction at work. When a skate moves over the surface of ice, the friction between the skate and the ice generates heat that melts the outermost layer of ice.
