Why did the bigbang occur?

Why? I think because of the singularity which is infinitely dense,infinitely hot and infinitely small and hence infinitely unstable hence the big bang for stability

On a more complicated note the universe was once in the form of a false vacuum, highly symmetrical but also highly unstable- take the analogy of trying to use a small sheet to cover an over sized bed, sheet may cover up but ones you lie on it is wraps ups again hence d meaning of false symmetry or false vacuum

So to attain true symmetry and stability the big bang is required( a formal of symmetry breaking)