The Big Bang model includes no such claim. The model deals with the expansion of the universe from a very early stage. Before that, the hypothesis is that it started from an initial singularity at which both space and time began. But since our laws of physics cannot be applied back beyond a certain point, it is not possible to make that extrapolation with confidence. If there was indeed an initial singularity, why that was so and why expansion occurred cannot be known. At least, that is my understanding of the theory.
Like all theories in science, it has a certain scope of application. Beyond that, while it may be easy to ask further questions, the theory cannot be expected to provide the answers to them.