While being made, the cereals are cooked into dry, crispy froths of syrupy, starchy stuff. In the process, and on cooling, they harden, and stresses induced during drying distort them. If you let them slowly absorb humidity from the air, they soften and the stresses relax gently, evenly and quietly. The cereals go leathery and sullen, and pass gently into milk and stomach.
However, if you wet the fresh, crisp cereals their stresses are relieved suddenly and unevenly and thousands of minute shells suddenly find themselves with one side hard and stressed and the other side soft. They are bite.
The processing of cereals for some breakfast foods creates bubbles in what is a brittle but weak material which swells when it absorbs water. As liquid touches one side of the bubble it swells and distorts so that the opposite side splits.
The sound of this splitting is amplified by the air cavity in the same way as a stringed musical instrument vibration is amplified by its sound box.