In QWC the cause of the Big Bang was:

So here is the cause of the Big Bang; but of course if there was a cause of the Big Bang then we will have to rename Big Bang Theory. The new name will have to represent a cosmology that enables our big bang universe but which also explains the “before” and “beyond” the Big Bang. So first, why do I think the Big Bang even had or needed a cause? It is not so much that Big Bang Theory doesn't mention the cause, which it doesn't. It is more that the law of inertia requires a force to set the universe into motion. So I think the expansion that we observe needed a force. That force seems to have been applied to whatever composed our arena which is now the observable universe. It was a force that occurred before BBT picks up and goes into inflation and nucleosynthesis. General Relativity Theory, a pillar of BBT, backtracks to a zero volume universe if you go back 13.7 billion years ago. And if you consider the mass and energy now displayed in the observable universe you have to realize that it would have all been contained in that zero volume of space. That means that GR implies a start point of zero volume and infinite density that is referred to as a singularity. It is called a singularity because the math, and GR is a mathematical model, fails at the start point. That is why BBT starts the instant after that singularity would have come into our “look back”. The theory calls it an event horizon and we just can't look back to before that point in time using GR or BBT as described by Sean Carroll in this blog at Cosmic Variance. Many possibilities have been explored. The most believable are models that deal with a universe before the big bang as one of decreasing entropy. Without some unforeseen changes, the entropy of our current known universe follows the arrow of time forward to complete entropy where there is no useful energy left and life and circumstances as we know them have long since dissolved into an infinite fineness, referred to as a de Sitter universe (Note: De Sitter, one of Einstein's associates, predicted dark matter and developed a solution to Einstein's field equations that assumed a universe with a positive expansion and only dark matter.) One possibility mentioned by Carroll is Quantum Loop Gravity (LQG). The math of LQG, one of the reverse entropy models with math that describes a universe before the Big Bang, is able to avoid the singularity of General Relativity Theory by defining a finite universe at the instant of the big bang. This approach describes a big "bounce" that marks the end of a period of reverse entropy that preceded our expanding universe. Critics argue that this bounce either seems too convenient or too inconvenient as interpreted from Sean Carroll's response to a blog at Bad Astronomy by Phil Plait. I include his quote below but first, the important thing about LQG is the math that accomplishes the astonishing feat of surviving the singularity produced by the math of GR. The disappointing thing about this reverse entropy model is the uncertainty about what such a prior universe might be like. I didn't use the word uncertainty just to imply that we aren't sure, which of course we are not. I used the word uncertainty because it appears that the reason Carroll and I'm sure others have trouble with the reverse entropy cosmologies in general and LQG in particular is because of the Heisenberg Uncertainty Principle which has become a pillar of modern quantum mechanics. Maybe the Uncertainty Principle is necessary to explain the quantum realm; I see how it could block the road to reason beyond BBT. It blocks the vary reasoning that we will need more than ever to work our way back into the reverse entropy models. It threatens to apply that same road block that BBT applied for several decades that kept science from breaking the Big Bang singularity barrier to reach reverse entropy models. It says that by breaking the singularity barrier we enter a universe that existed before our universe and concludes that because of the uncertainty principle applied to the bounce point of minimum entropy, we cannot be sure of anything about what such a prior universe might be like. Sounds the same as we heard from BBT, "Why talk about before the Big Bang since we can't know anything about it due to the event horizon", doesn't it? In Quantum Wave Cosmology the cause of the Big Bang was the burst of a big crunch. Does that make QWC a reverse entropy model? Yes. But that does not mean that QWC is disappointing like the LQG model as described by Sean Carroll. I mentioned this comment above, "The real problem with all such models is that, from the point of view of the other side of the bounce, the entropy is decreasing as the universe collapses, which seems crazily finely-tuned. Either that, or the entropy is at a minimum at the bounce, for no especially good reason. Singularities are going to have to be resolved somehow, but reality is likely to be quite a bit more complicated than simple bounce models." That is why Quantum Wave Cosmology exists. It solves the entropy problem without being fine tuned at all in the sense of what causes the collapse into a big crunch. And there is good reason why entropy at the bounce is at a minimum. QWC is not a simple bounce model, and not simply a reverse entropy model. So this is where I scrap that particular application of the Uncertainty Principle. This is where I let reasonable and responsible speculation take over from it. This is where we go not "before" the Big Bang, not back into a "prior" universe, but out into the greater universe of Quantum Wave Cosmology within which our expanding universe is just a tiny and temporary arena in a greater universe, an infinite universe that has always existed.