We don't know what caused it but we are working on it

Because there is no earlier, the big bang marked the beginning of time too (in this universe)

Since we don't know what caused we don't know the mechanism either although an explanation could be the collision between the membrane of our universe with another going by string theory

We are guessing it did happen somewhere else in the multiverse

You can say nothing since time (in this universe) began with the big bang