According to research at Lawrence Berkeley Labs about zirconium titanium glasses, the answer is: "It's the club, but we are not sure why."

These titanium clubs are made of zirconium titanium glass. The impact heats the metallic glass very rapidly, and once it is hot it oxidizes rapidly, which is what causes the sparks. Why it heats so rapidly is not entirely clear, but it appears to be related to a combination of hardness and low thermal conductivity. The hardness enables large stress to build up in the material before any structural failure occurs. When failure does finally occur, the stress buildup is converted into heat. This then combines with the low thermal conductivity of the metallic glass to produce local temperatures that are white hot (~1100 C), the temperature that Berkeley researcher Robert Ritchie observed in a nitrogen atmosphere. It seems that even higher temperatures are reached in air, which may be related to a chemical reaction between the metal and oxygen.