QUOTE
Friction Material Fade – When pads reach high temperatures they can sometime “outgas” chemicals that redeposit themselves on the surface of the brake pad. This decreases the coefficient of friction and results in a decrease of brake power. The lever will remain firm however the brake will not generate the normal amount of power. This is also described as when pads “glaze” over.
It's all I could find. Maybe if you google the materials that your pads are made from, you could find out what they change into when heated. Just a thought.