Why time begin at the Big Bang

The Big Bang was the granddaddy of all vacuum fluctuations and it was caused by the fact that the space-time continuum had just started. At the very moment that time began to exist, there was almost zero uncertainty in time (the uncertainty in time can't be larger than the amount of time that has existed).
The Heisenberg uncertainty principle provides a quantitative relationship between the uncertainties of the hypothetical infinitely precise measurements of p and x as measured by the sizes of their distributions in the following way: If the particle state is such that the first measurement yields a dispersion of values Δx, then the second measurement will have a distribution of values whose dispersion Δp is at least inversely proportional to Δx.
The other formulation of the Heisenberg uncertainty principle says that the product of the uncertainties in time and energy has to be at least as large as planks constant divided by 2 pi.
dT*dE >= h/2pi
dE >= h/(2pi*dT)
So if dT is very, very tiny (nearly zero), then dE is very, very large (nearly infinite).
A nearly infinite uncertainty in energy requires a nearly infinite amount of energy available. That much energy makes a bang....a very BIG BANG.
Cause and effect are properties of time. If there is no time, then cause and effect do not exist. So, the substrate for cause and effect (time) cannot have a cause. Asking: "what caused the Universe" is like asking: "What is the square root of green?” It has all the right parts of a question, but it is completely unintelligible.