We don’t know. The physics we understand breaks down at the Big Bang. We can only explain the Universe back to a time about 10^-43 seconds (that’s a 0.000….043 – with 43 zeros in total) after the Big Bang. Before that gravity just couldn’t work in the way we understand it so all our models break down.
So basically right now your guess is as good as mine (or even Brian Cox’s) as to what caused the Big Bang to happen.
We don’t really know! All we know is that suddenly, nearly 14 billion years ago, everything that is in the universe today expanded out unbelievably quickly from a tiny unbelievably dense spot. I find just thinking about that scary enough, without thinking about how it might have been caused!