Why did it happen?
We can never know the answer, because any possible cause was outside of our observable universe. Inside of our universe, time started at the big bang, so there was no 'before' and thus there was no cause, because the cause needs the time to be before the effect.
[0] https://www.independent.co.uk/space/universe-expanding-colla...
This is true for anything we don’t know.
For the Big Bang, we know the intermediate problem: black holes. Gravity and quantum mechanics interacting at the same scale. The singularities there are accessible in a way the singularity of the Big Bang is not.
I mean, obviously this is nonsense, but... I can't exactly disprove it.