Fat utilization/beta-oxidation of fatty acids is slower at providing energy than carb utilization/glycolysis. Therefore the ratio of carbs to fat utilization increases at higher exercise intensities. If you ride slowly enough, your body uses a high proportion of fat versus carbs, and it's estimated that you could ride a month off the energy in the body's fat stores (hypothetically only!).

Once the intensity of exercise is increased, the proportion of energy derived from carbs starts to go up to meet the increased rate of demand for energy. There is a limited amount of carbs stored in the form of glycogen in your muscles and in your liver. It is believed that your body has some sort of "glycostat" to keep the glycogen from dropping to a life threatening level (the brain requires glucose), and that's why your body bonks: to terminate high intensity exercise which is drawing the glycogen/carbs tank down too close to empty and forcing you back into burning your basically inexhaustible fat stores.

With endurance training, athletes can improve the speeds they can ride at the same carb to fat utilization ratio. Some people think you can train the utilization ratio thru adjustment of diet while exercising and/or at rest, but as seen time and again, the actual training program has the greatest magnitude of effect on performance; therefore I personally believe many people neglect to train well and dwell too much on factors which are more akin to icing on the cake or the cherry on top. What good is it to worry about having the most perfect cherry on top if you didn't work hard enough churning to make enough ice cream for the foundation of the ice cream sundae?