What causes chemical bonds to break during a reaction?

Chemical bonds are broken by the energy that is released during the chemical reaction. This energy is created by the movement of electrons from one molecule to another as they attempt to achieve a chemical balance.

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.