If all of the mass and energy in the universe was in a point smaller than the size of an electron at one time, why did it expand? Did it necessarily expand immediately on formation, or could it have remained in an initial state for some length of time until some phase threshold was reached?
It's an important question, because it tells us something important about the Big Bang model. Some think that the Big Bang explains the expansion, but actually, it takes expansion as an initial condition (and it is typical of dynamical models that they require an initial condition that is outside the model) and tracks how it proceeds. We generally don't explain why our models work, we just test our models. We would need some new model to test, one that currently does not exist or is highly speculative, to be able to say why the Big Bang model works, and why the universe is expanding.