Math is a language. It expresses things. Are you able to speak, in English, about things that could possibly have happened before the Big Bang? You are? Well then it shouldn't surprise you that math can do the same thing.
While it is technically correct that there was nothing "before" the Big Bang, you have to remember that this is being measured in terms of space-time. Space-time was created at the Big Bang, so obviously in those terms there was nothing "before" this happened. That doesn't mean the Big Bang can't have a cause, it just means that this cause was outside our current model of space-time. A good example of something outside that model would be the hypothesis about the Singularity (and by extension the Big Bang) coming from the collapse of a 4-D star.
The point is that math is not limited to modeling space-time, so it really shouldn't be a surprise that it's used to try and model things that came "before" the Big Bang.
