If the Big Bang wasn't brought into existence by free will, then there must be an infinite regress of preceding causes that led to up to the Big Bang.
Given an infinite amount of time, any action that has a greater than 0 chance of occurring will inevitably occur.
One this action inevitably occurs, we can count backwards the number of trials that led up to the action. By doing this, the entire event has a quantifiable beginning.
The problem is that given an unlimited amount of time, something that can happen inevitably will. This gives you an infinite chain of events that all have quantifiable beginnings to them which is logically absurd. The only way around this is if an action occurred by free will - in that case the event will not have a quantifiable beginning.